Combine the 400 g fresh milled flour and 100 g bread flour in a large bowl. Pour in 350 g water, stir until no dry bits remain, cover, and rest 30 minutes.

Add the 100 g starter and 10 g salt, mix with wet hands until evenly distributed, cover, and rest another 30 minutes.

Bulk ferment 4 to 5 hours at room temperature, doing 4 sets of stretch and folds during the first 2 hours, until the dough doubles and jiggles.

Shape the dough into a tight boule on a lightly floured surface, rest 10 minutes, then place seam side up in a floured banneton.

Preheat Dutch oven in a 475 °F oven for 30 minutes. Turn loaf onto parchment, score the top, and bake covered 30 minutes. Uncover, reduce heat to 450 °F, and bake 12 to 15 minutes until deep amber.

Cool on a wire rack at least 1 hour, then slice and serve.