Quelle is the wrong question word as quelle always requires a noun:

Quelle fougasse tu as mangée? -- Which focaccia did you eat?

If you want to know what a fougasse is, you use one of the methods outlined in the lesson, for example:

Qu'est-ce qu'une fougasse? -- What is a focaccia?

Thank you for the great explanation Chris!

Actually, I had one follow up question.

When I took French classes in school, my teacher would always ask "Quelle est la date?"

Do you mind explaining why that question is correct but not in other contexts?