When it comes to the agreement of the verb with a compound subject (containing more than one word), the rule is that what noun which is the nearest to the verb will agree with it. So if the noun directly in front of the verb is singular, then the verb will be singular as well; if it is plural, the verb is plural too.
Here, we have to complete the sentence 'is for dinner tonight'. The verb is is singular, which means we have to have a singular noun directly in front of it. The word spaghetti is treated as a singular noun in English, which means the correct answer is Enchiladas or spaghetti.
Enchiladas or spaghetti is for dinner tonight.