Learn Portuguese (Portugal): Às vezes eu como no restaurante.

It answers quando (when). You wouldn't use it to answer a que horas (at what time) because Às vezes means "sometimes," which is a general frequency, not a specific time on the clock.

Why does the "A" in Às vezes have a backwards accent?

That is called a grave accent, and it shows that two words crashed together. Here, it is the preposition a (at/to) combined with the feminine plural article as (the). So, Às vezes literally translates to "at the times".

In the question a que horas, we use horas for time. Why do we use vezes for "times" here?

English uses "time" for two different concepts, but Portuguese separates them! Use horas for clock time (like asking a que horas). Use vezes (the plural of uma vez) for instances or occurrences, like "sometimes" (às vezes) or "three times" (três vezes).

Why do we use no before restaurante instead of just em (at)?