Translation:There is onion, tomato, cheese, and sausage on the pizza.
They are now. (Well, in some weeks or days when the change shows up.)
I'm not changing the singular ingredients to plural, despite the suggestions in this thread, since it's not wrong to have onion and tomato on one's pizza, and it would just make the translation back into Finnish unnecessarily difficult.
"There is" is correct here (logically it shouldn't be... but English is just mean like that). Starting a list with "there is/are" you conjugate to the plurality of the first item. E.g. "There is a table, bed, and chairs in my room" or "There are chairs, a table, and a bed" but never "There are a table..."
There is no problem with listing without articles (the optional article here would be "some" as we are treating the onion as a mass noun) it may sound strange to you saying it aloud because we would always contract "There's" in this sentence, but that's stylistic not grammatical.
The only problem here is "in" when it should be "on"