Preheat oven to 180°C (350°F) and grease a baking sheet.

Heat olive oil in a heavy-bottomed pan over medium-high heat.

Add sliced onions, salt, and rosemary to the pan and stir.

Cover and cook until softened, about 10 minutes, stirring occasionally.

Reduce heat to medium-low and cook until golden brown and caramelized, about 20 minutes longer, stirring occasionally to prevent burning.

Remove from heat and let cool.

Flour a work surface generously.

Place chilled pie crust on the flour and flatten it with your palms, stretching it outward.

Roll out the dough to a large round of about 1/4 inch thickness.

Transfer the pie crust to the greased baking sheet.

Spread the caramelized onions on the pie dough, leaving a 1 1/2-inch border.

Sprinkle with more rosemary (optional).

Fold the edge of the dough up and over the filling.

Bake for 30-35 minutes, until the dough is richly browned.

Sprinkle with ricotta cheese and red chili flakes.

Cut into wedges and serve warm.