Curry lives and evolves, diverging and diversifying each time a cook makes her own version. It's possible to trace some curries not only to the region of, for example, India or Thailand they originated from, but also to specific villages, in some cases villages that existed in the 17th century. The same idiosyncratic phenomenon occurs when you make your own curry sauce. Curry sauce is itself a type of curry -- a natural result of liquids, aromatics and spices merging to form a preparation unique to the ingredient combinations and cooking techniques you use -- and consists of the same components: flavor base, body, thickener and finishing ingredients.
Curry sauce's flavor base consists of standard aromatics and spices: the combination of ginger, garlic and onions, a recognized regional variant of mirepoix, and curry spices, which come dried and in paste, or masala, form. Curry pastes and spice mixes are intrinsically linked -- to make the former you must first make the latter. There isn't a universal curry-spice combination -- ingredients vary by cook and region -- but certain spices are typically found in all variants.
Combine dried turmeric, coriander, fenugreek and curry leaves to make curry-spice base. Then add a few or several of the following spices to taste: cardamom, chili powder, fennel, allspice, mustard seeds, asafoetida, cinnamon, cloves and black pepper.
Heat the curry spice over low heat in a saute pan until fragrant. Add 3 parts chopped onions and 1 part each chopped garlic and grated ginger to the oil. For a bold curry sauce, adjust the heat to medium, and saute the aromatics and spices until dark and caramelized. For a mellow curry, continue cooking the ingredients over low heat until they soften. Use approximately 1 cup of aromatics for every 2 cups of curry sauce.
Curry sauce derives body from one or more of the following ingredients: water, stock, yogurt, cream or coconut milk. Consider a tempering ingredient -- yogurt, cream and coconut milk -- for the body if you like spice without the lingering burn that follows. Use water or stock for a lighter consistency.
Add the body liquid and simmer it until it reduces by about a third. Season the curry sauce to taste with kosher salt after it reduces.
Reduce the body of the sauce by two-thirds to make a paste. Store paste with a layer of oil on top of it in a container in the refrigerator until you're ready to use it. Thin paste using stock or water when you're ready to proceed with the sauce.
Vegetables give curry sauce viscidity. Tomatoes, lentils, onions and spinach, to name a few common examples, invest their own characteristic taste and textural qualities to the sauce. Lentils, for example, absorb flavor more readily than spinach, but spinach results in a lighter consistency. You're not limited to one thickener, either; you can add a combination of vegetables to taste.
Puree 1 cup of vegetables for every cup of sauce. Add the pureed vegetables to the sauce and simmer them until the sauce reaches the desired consistency.
Taste the curry sauce to determine what, if any, finishing ingredients it needs. Finishing can be as simple as adjusting the salt and pepper or adding a touch of cream or butter for richness -- it ultimately comes down to preferences. Add a pinch of sugar to contrast the spices, or a squeeze of lime or lemon juice for acidity. Stir in a bunch of cilantro or parsley and let them heat in the sauce.