How to Make It
Heat oven to 400° F. Heat the oil in a large pot over medium heat. Add the onions and ¼ teaspoon each salt and pepper. Cook the onions, stirring occasionally, until golden brown and completely soft (adding 1 to 2 tablespoons water, if needed, to scrape up any browned bits), 15 to 20 minutes. Add the broccoli, half the Feta, and the rosemary.
Meanwhile, roll the piecrust on a floured surface into a 14-inch circle. Place on a parchment paper–lined baking sheet. Brush the crust with the mustard, leaving a clear 1½-inch border.
Top the piecrust with the broccoli mixture, leaving the same border. Fold the edges of the piecrust over toward the center, overlapping slightly and covering the filling by about 1 inch. Top the filling with the remaining Feta.
Bake until golden brown, 30 to 35 minutes. Serve warm, with the salad.