|In the course of cooking Indian food, I've found that many dishes have a great number of similarities. This is a distilled version of what they have in common.
Since not every cook has every ingredient, and there's no accounting for taste, I've put an asterisk (*) next to the ingredients and steps that are required. The rest can be considered optional.
Prep Time: 15 Minutes
Cooking Time: 15 Minutes
* 2 tbsp vegetable oil (canola works best) 
1 tsp cumin seeds 
* 1 medium red onion, chopped 
* 2 cloves garlic, diced 
* 1 inch piece of ginger, peeled and diced 
1 or 2 green chiles, seeded and diced 
* 1 tbsp cumin powder 
* 1 tbsp corriander powder 
1/2 tsp chile powder or cayenne pepper 
* 1/2 tsp turmeric 
* 1 can of petite-diced or crushed tomatoes 
something else 
2 tbsp fresh, chopped cilantro 
* Heat the oil in a sauce pan, wok or pot until it gets really hot (i.e. when a drop of water hits the pan, it sizzles like crazy).
Add the cumin seeds and fry until they start to brown.
* Add the onion and stir well. Mix it up occasionally. When the onion starts getting soft and translucent, it's cooked enough.
* Add the garlic, ginger and chiles. When the garlic turns light brown, move along.
* Put in the spices (do not use the yellow supermarket "curry powder"; that stuff is too bland and is to "real" curry powder what Pizza Hut is to "real" pizza), stir well, and cook for another minute or two.
* Pour in the tomatoes and stir occasionally. You want to make the tomatoes' water evaporate, so the sauce is thicker. Aim for something roughly as thick as tomato sauce; not too watery and not too thick and pasty.
* Add your anything ingredient. This can be range from canned red beans to zucchini to pieces of leftover chicken. Stir well, to let it absorb the ingredients. If the sauce starts gets too dry, just add some water and mix well.
Add the cilantro as garnish (cooking destroys a lot of its flavor and fragrance) and salt and pepper to taste.
Serve with long-grain rice (e.g. basmati), plain yogurt (or raita) and chutney.
So, there you have it — a typical "curry" sauce is just (1) onion, garlic, ginger and chiles that have been chopped and fried (2) with spices mixed in and then (3) some mushed tomatoes added.
Indian food is delicious, but don't let it intimidate you — from Rogan Josh (some cardamom) to Madras (lots of chile) to Vindaloo (vinegar and lots of chile) to Tikka Masala (lemon juice and more turmeric), most Indian dishes you're familiar with are just nuanced variants on the above.
 Canola oil is healthy, has a fairly high smoking point and doesn't alter the flavor of ingredients.
 Cumin seeds, especially when roasted or fried, add a nice earthy flavor to the food.
 White or yellow onions will work fine, if you have them. You add the onion first, because it has a longer cooking time than garlic, ginger and chiles and, if you added them simultaneously, you'd get undercooked onion or burnt garlic.
 Fresh garlic is best. Garlic paste is OK. Try not to use those jars of pre-chopped garlic; they have less flavor.
 Use fresh ginger, if possible. Ginger paste is all right. Powdered ginger and jarred, pre-sliced ginger are no good. That pink, Japanese-style pickled ginger couldn't be more wrong for this dish.
 I've used Jalepeño, serrano, and Thai birds-eye in curries. Any kind is fine, except habañero, which is too spicy (picture your intestines catching on fire). If you're feeling brave, you can leave the seeds and pith (the white part that holds the seeds together) in.
 Ideally, this is dry-roasted and then ground. But a jar of cumin is fine as long as it's not too old (ideally under 6 months since you bought it).
 Same as cumin, above, but there's no need to roast.
 Paprika doesn't have enough flavor, even if it is, technically, a ground red pepper. I actually use a full teaspoon.
 Turmeric adds that beautiful yellow color to Indian dishes. Don't use too much (e.g. a tablespoon) or it will give a pasty texture to the dish. It has a slight scent and flavor, but for some reason, it seems to make Indian food easier to digest. There are various reputed medicinal properties but, from what I gather, you'd have to consume turmeric quite frequently to gain any measurable benefits.
 You can use fresh tomatoes, but when you're adding this many other ingredients and cooking thoroughly, it doesn't make as much of a difference as it would in a salsa or pico de gallo. Diced tomatoes will make it slightly chunkier than crushed.
 A wide range of things will work here: (already-cooked) chicken, white fish, canned kidney beans, canned chick peas, canned black-eyed peas, string beans, zucchini ... almost anything you have around. Experiment freely, but you'll ideally use something that doesn't require much cooking (sliced zucchini) or has already been cooked (chicken).
 Cilantro is a love-it-or-hate-it herb that is used to enhance the flavors of other ingredients. If you cook cilantro, it loses most of its flavor and aroma. Also, some people find it has a gross, soapy taste (it's a genetic thing, not a matter of unfamiliarity), so it's entirely optional.