Add 1 tablespoon olive oil, sugar, flour, and salt. Stir until a soft, sticky dough forms.

Drizzle oil on the dough and your hands. Stretch dough into a smooth ball. Cover and refrigerate overnight.

Let the dough rise at room temperature until doubled in size, about 1.5 hours.

Oil a work surface and transfer dough. Press into a rectangle, then fold into thirds.

Oil a sheet pan and transfer dough rectangle. Let proof until spread out and bubbles form, 1.5 to 2 hours.

Wrap plum sections with prosciutto and refrigerate until needed.

Preheat the oven to 475°F (245°C).

Sprinkle dough with rosemary. Push wrapped plum sections into the dough. Drizzle lightly with olive oil and sprinkle with sea salt.

Bake until browned, 17 to 20 minutes. Brush with more olive oil if desired. Cool before cutting.