Ingredients and heat control — neither can be skipped.
Tips
grammar
Usually sentence-final, summing up a list that came before: '(X, Y, Z), 缺一不可.' Not used in isolation; always refers back to a set of listed items.
history
From 《水浒传》 ch. 20: Lin Chong insists the strategist must join the council with 鼎分三足,缺一不可 — 'like a tripod with three legs, not one can be missing.' The tripod image is exactly right: take away any leg and the whole thing falls.