Enchiladas or spaghetti is for dinner tonight.
In a sentence, when one subject is in plural ("enchiladas") and the other is in singular ("spaghetti"), and they are connected by the words "or", "nor", "neither/nor", "either/or", or "not only/but also", the verb must agree with the subject that is closest to it. In this case, "spaghetti" is the one that is closest to the verb, thus, the verb must be in singular as well ("is").