Learn German: Das Essen war nicht schlecht, im Gegenteil. - The food was not bad, on the contrary.

Prepositions rarely translate perfectly between languages. While English says "on," German uses "in." The noun is neuter (das Gegenteil), and because it is in the dative case here, in dem contracts to im. It is best to memorize im Gegenteil as a fixed expression.

Essen is a verb meaning "to eat", so why is it capitalized here?

In this sentence, Essen is being used as a noun meaning "the food" or "the meal." In German, you can turn any verb into a neuter noun (a process called nominalization) simply by capitalizing it and adding the article das.

Why does the sentence use the simple past war instead of the conversational past ist gewesen?

Although spoken German typically prefers the conversational past (Perfekt) for most verbs, the verb sein (to be) is a major exception. Native speakers almost always use its simple past form, war, even in casual, everyday speech.

Could I move im Gegenteil to the front of the sentence?

Yes, you could say Im Gegenteil, das Essen war nicht schlecht. When used at the start like this, it acts as an introductory phrase separated by a comma, so it doesn't change the standard word order of the main sentence.