rosti

One root, and it needs saying. Ros- looks like it might be doing something - Esperanto has roso (dew), and a learner in affix-hunting mode can talk themselves into almost anything. It is not that. Rost- is a single Germanic root borrowed whole, roso is unrelated, and the -i at the end is nothing but the infinitive ending. When a short verb resists decomposition like this, the honest move is to memorise it rather than to build a story around it.

Roast, grill, toast - one word for three. English splits cooking-with-dry-heat three ways depending on the equipment. Esperanto uses rosti for all of it and specifies only if it matters: rosti en la forno is roasting, rosti super fajro is grilling or barbecuing, rosti panon is making toast. Rostita pano is toast; rostaĵo is a roast, the joint of meat itself; rostilo is the grill or toaster, the -il- tool suffix doing its usual clean job.

The cooking verbs, sorted. Kuiri is the general one, to cook. Baki is to bake, and belongs to bread and cakes. Boli is to boil, friti to fry in fat, rosti is dry heat. Kuirejo is the kitchen and kuiristo the cook.