How do you write a letter to someone you love telling them your feelings for them is like an onion this is for an English hw?

Don't, its a bad idea. Who thought of this HW assignment?! When you pull away the layers,it gets smaller and more ripe. You also cry a lot. Just like feelings.

you could say you love how they have layers like an onion or something, and how much you want to share with them and stuff