Scatter the sugar over the bottom of a large skillet. Cook over medium heat until it melts and turns light amber, 4 to 5 minutes. Add the onions, toss to coat, and cook until deeply browned, 20 to 30 minutes, stirring in a splash of water if the pan goes completely dry. Remove the pan from the heat and set it aside.
Put the broccoli rabe and 3 tablespoons of water in a medium skillet over medium heat. Cover the skillet, and cook the broccoli rabe until it turns bright green, 2 minutes. Uncover the skillet, and cook until the water evaporates, 1 to 2 minutes. Add the oil, garlic, red pepper flakes (if using), and 1/8 teaspoon of the salt. Cook until the broccoli rabe is tender, 2 to 3 minutes. Remove the pan from the heat and set it aside.
Heat the oven to 400ºF. In a bowl, combine the ricotta, ¾ cup of the mozzarella, the oregano, and remaining 1/8 teaspoon of salt.
Unfold the pastry on a lightly floured surface. Cut it in half crosswise, dust each half with flour, and roll it into a 7- by 6-inch rectangle. Use a spatula to transfer each rectangle to a baking sheet. Brush all around the edges of the pastry with water, and fold the edges in by ½ inch, pinching to create a crust. Prick the center of the dough all over with a fork.
Spread the ricotta mixture over the dough, and top with the reserved onions and broccoli rabe and remaining ¼ cup of mozzarella. Bake until the pastry is puffed and golden brown, about 20 minutes. Cool the pizzas on a baking sheet for 10 minutes. Cut each into 8 wedges.
Tip: If you can’t find broccoli rabe, use broccolini. Or use Swiss chard or turnip greens and reduce the cooking time by a minute or two.