Put an inverted baking sheet or pizza stone on the top rack of the oven. Preheat the oven to 400 degrees F. Lay a sheet of parchment on another inverted baking sheet and sprinkle with cornmeal.
Form the pizza dough into a large rectangle, about the size of the baking sheet, and put on top of the parchment. Cover the dough with the sliced onion and season with 1 teaspoon salt, a few grinds of pepper and 1/4 teaspoon red pepper flakes. Sprinkle with Parmesan and drizzle with 3 tablespoons of the oil. Slide the parchment onto the hot baking sheet. Bake until the dough is fully cooked and the top is golden brown, about 25 minutes.
Heat 1 tablespoon of the olive oil in a large skillet over medium heat. Add the chopped onion, the garlic, paprika and the remaining 1/2 teaspoon red pepper flakes and cook, stirring, until soft and they begin to caramelize, about 4 minutes. Stir in the tomato paste and ground beef, sprinkle with salt and cook, stirring, until the beef is browned and cooked through, about 4 minutes. Add the collard greens, lemon zest, lemon juice, and 1/4 cup water and cook until heated through. Season as needed.
Spread the beef and collards evenly over the flatbread. Sprinkle with additional cheese, drizzle with the remaining tablespoon oil, and cut into squares or strips.
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