When we talk about dividing food among a few people, we normally use "slice" just before the cutting (e.g., "Would you like a slice of cake? / I'll have a slice of pie, please."). Later, in discussing the meal, we'll usually use "piece" (e.g., "I had a big piece of birthday cake last night").

I wouldn't say 'Piece of tomato' but have difficulty figuring out why... It's definitely If in doubt use 'some' 哈哈 to avoid the dilemma. Look at the meaning of slice: 'a tin broad piece or wedge cut off. CUT is the key word. If it's not 'cut' (literally or metaphorically) it's not a slice. Hmmm How about a slice of the action - that's where you take 'your cut'. But if it's easy it's a 'piece of cake' 555. Ok, I've said my piece.