There are some ingredients though, that are common in most Indian dishes though and preparing them ahead of time can further cut down on cooking time.
Chopped fine, sliced thin or ground to a paste, onions form the base of gravies, a complement to veggies or an important ingredient in salads. Chop, slice and grind them and freeze in labelled freezer bags. That way when a recipe calls for onions you have them quickly at hand.
Ginger and garlic pastes:
Ginger and garlic are also vital ingredients in Indian cooking. A recipe may require them chopped fine or ground to a paste. I always have a large glass bottle each of ginger and garlic pastes in my refrigerator. You can just as easily buy them from the grocery store (Asian or Indian groceries will definitely have them) but I like to make mine at home so I can be sure they're free from preservatives. Here's a handy tip for keeping ginger and garlic pastes fresh for longer. I also store peeled garlic in a date labelled box or Ziploc bag for when a recipe requires it to be chopped fine.
Again this is an ingredient you can quite easily buy, but it always seem to taste better when you make it at home. Make tomato pureé ahead of time and pour into ice-cube trays (I always use a tablespoon to measure how much fits in a single cube so that its easier to measure out as the recipe requries later) and freeze. When frozen pop them out and store in date and measurement labelled freezer bags for when you need them.
Fresh herbs like coriander and mint:
Coriander and mint are used to make chutneys, added to gravies and as garnishes on dishes and salads. With Asian and Indian food being so popular these days, these herbs can even be found at your local grocery store. They're not always in season though. Here's a handy tip for having them around whenever you need them, no matter what the time of year. Here's to herbs on demand!
A majority of the time when onions are used in Indian cooking, the recipe calls for them to be browned. Make up a batch ahead of time and store in your fridge. Make both chopped and sliced browned onions. Here's a quick tip for browning onions quickly.
Most gravied dishes have certain ingredients in common, so preparing and freezing one when you have some spare time is a great idea. Here's the recipe for a Basic Indian Gravy. When you're ready to cook use as is or add any extra ingredients as required.
Chapati/ Paratha/ Poori dough:
Breads like Chapati, Paratha and Poori are the perfect accompaniment to most Indian dishes. They're also great by themselves or with your favorite spread or filling. My favorite thing to do with them is to fill them (freshly made) with leftover cooked veggies or meats and make a roll! Look here for more ideas on how to use leftovers to create a whole new meal.
Dough made from wholewheat flour will keep really well in the fridge for 3-4 days so make in advance. Here's the recipe for the dough. You can also pre-cook and store Chapatis in the freezer so that they are almost cooked when you need them.
For more handy, time saving hints look at my Quick Tips section!