Mix the flour and water until no dry spots remain. Cover and let rest 30 minutes. Add the starter and salt, then mix into a shaggy dough.

Fold in the cubed cheddar and diced jalapeños until evenly distributed in the dough.

Cover and let rest 30 minutes. Perform one set of stretches and folds. Repeat three more sets every 30 minutes, for 2 hours total.

Turn dough onto a floured surface, shape into a round or oval, and rest 20 minutes. Shape tightly, place seam side up in a floured banneton.

Place a Dutch oven in the oven and preheat to 475°F (245°C) for at least 40 minutes. Let the loaf proof at room temperature until puffed. Turn onto parchment, score, and bake covered for 30 minutes, then uncovered 15 to 20 minutes.

Cool on a rack at least 1 hour, then slice with a serrated knife and serve.