Preheat oven to 425 degrees F (220 degrees C).

Heat olive oil in a skillet over medium-high heat; saute onion in hot oil until slightly softened, 3 to 4 minutes. Add rosemary and a pinch of salt; saute until onions are golden and soft, about 5 minutes more.

Spread cornmeal over the bottom of a baking sheet. Stretch pizza dough in a thin layer onto baking sheet over cornmeal. Spread onions onto dough and sprinkle black pepper over the top. Sprinkle feta cheese, pumpkin seeds, and Parmesan cheese over the top of the onions.

Bake on the bottom rack of the preheated oven for 5 minutes. Rotate the baking sheet 180 degrees and bake 5 minutes more. Transfer baking sheet to the middle rack of the oven and bake until bread is golden, crispy, and cheese is melted, about 5 minutes. Transfer flatbread to a wire rack to cool.