トマトと玉ねぎをのせてください。

tomato

if; when; and; with; particle used for quoting (with speech, thoughts, etc.); quoting particle; promoted pawn; indicates question (sentence end)

onion (Allium cepa)

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

to place on (something); to give (someone) a ride; to give a lift; to pick up; to help on board; to load (luggage); to carry; to take on board; to send out (on the airwaves, etc.); to deceive; to take for a ride; to (sing) along with (musical accompaniment); to let (someone) take part; to excite (someone); to publish (an article); to run (an ad)

to give; to confer; to bestow; to kindly do for one; to oblige; to favour; to favor