Way to Differentiate between Necessary vs Additional Ingredients in Recipes
Get a stack of 10 or 20 trustworthy recipes for a dish - and compare.

Also take advantage of ingredient slots that seem optional in one dish - sometimes they are a way to use up something that you bought for another recipe where it is not optional.

Classify ingredients by the role they play in the dish:


What do they do to the balance of the basic six tastes? Can the balance be kept by using another, more generic ingredient (eg add vinegar or lime if you leave out a sour ingredient)? 
Is there a functional reason for a sour/alkaline, salty, sweet, fatty ingredient? pH can matter to how something cooks; salt and sugar can interfere with liquid balance; sugar can be textural; fat can do heat transfer; fat or alcohol can put aromas into solution.
Is there an aromatic role - and is there something with a similar aroma?
Is there a textural role? If yes, is it ONLY textural because seasoning overwhelms the taste/aroma anyway?


Also - there are a lot of "exotic" ingredients that exist across ethnic cuisines in a similar form but under different names - though sometimes salt/fat/sugar balances can be different. One example would be korean doenjang and japanese dark miso. Experimentation might be needed at first.