Gosh, you can put just about anything on puff pastry and it'll taste good.
This is essentially a sort of puff pastry white pizza. Two puff pastry sheets, smeared with ricotta cheese, topped with grated cheese and caramelized onions, topped with slightly sauteed mustard greens and baked. We ate it as a main course, but it would work well as an appetizer, too.
Two sheets of puff pastry, thawed enough to unfold
Grated gruyere, comte, or good cheddar cheese
Caramelized onions (about 1 big onion)
1 head of mustard greens, roughly chopped
Saute the onions in olive oil until they begin to color; then lower the heat to medium-low and let cook down for at least 30 minutes, until dark brown. Don't rush this step--the darker they are, the better they'll taste.
Open the two puff pastry sheets on a parchment-paper lined baking sheet, and smear each with ricotta cheese (maybe 1/2 cup per). Add the caramelized onions. In the onion pan, add a little more olive oil and quickly saute the mustard greens, maybe 5 minutes, until they start to wilt. Sprinkle grated cheese over the onions (maybe 1/2 cup per), and top with the mustard greens.
Bake at 400 for 20-25 minutes. Serve hot.