In a large mixing bowl, dissolve the Levain and the Active Sourdough Starter in about half of the Water until mostly smooth.
Add the bulk of the Bread Flour together with the Whole Wheat Flour and the Rye Flour, then mix until a rough, shaggy dough forms. Cover and let rest for an autolyse of 30–60 minutes to hydrate the flours and develop flavor.
After the autolyse, add the remaining Water if needed to reach a cohesive dough, then sprinkle in the Fine Sea Salt and incorporate thoroughly by pinching and folding until the salt is evenly distributed.
Perform a series of stretch-and-folds in the bowl: lift and fold the dough over itself every 30 minutes for 2 hours (about 4–5 sets). The dough should become smoother and stronger with each set.
Allow the dough to bulk ferment at room temperature until it has increased noticeably in volume and shows bubbles on the surface (typically 3–5 hours depending on temperature).
Turn the dough out onto a lightly floured surface, pre-shape into a round, and let rest uncovered for 15–20 minutes. Then perform a final shaping into a tight boule or bâtard and place seam-side up into a well-floured banneton or bowl.
Cover and proof in the refrigerator for an extended cold fermentation (8–12 hours or overnight) for the best flavor development—alternatively proof at room temperature until puffy but not overproofed.
Preheat your oven to 450–475°F (230–245°C) with a Dutch oven or baking stone inside. When ready, carefully invert the loaf onto parchment, score the top to control expansion, and transfer it into the preheated Dutch oven.
Bake covered for 20–25 minutes to build steam and set the crust, then remove the lid and bake an additional 15–25 minutes until the crust is deep golden-brown and the loaf sounds hollow when tapped (or reaches about 208°F / 98°C internally).
Allow the Pain de Campagne to cool completely on a rack for at least 1 hour before slicing so the crumb finishes setting and the flavors fully develop. Enjoy with butter, cheese, or your favorite spreads.
