Here, class is the oblique object of to. The oblique object is a complement of the preposition and hence it functions as an argumentsof the verb. Note that there are two complements in the following ...

But in grammar, it’s more specific. In the case of object complements, the word “complement” refers to something that completes the object, making the sentence a whole thought.

In this case, he explains, “slice” is a verb, “the onions” is the object of the verb, and the word that follows is the object complement — that is, a word modifying the object.