To prepare the onions: Place the onions, milk, and bay leaves in a saucepan. Bring to a boil, then lower the heat and simmer for 5 minutes. Remove from the heat, strain, and cool to room temperature. Discard the bay leaves, and reserve the milk and onions.
To make the dough: Put the cooled milk in a mixing bowl with the starter. Add the whole wheat flour and 1 cup (120g) of the bread flour. Stir in the onions, salt, yeast, and another 1 1/4 cups (150g) of bread flour. Cover and let sit for 10 minutes. If the dough is still sticky, stir in the remaining 1/4 cup (30g) of bread flour.
Knead the dough on an oiled surface for 10 seconds, return to the bowl, and cover. Let the dough rest for 10 minutes. Repeat the brief kneading for 10 seconds more, return to the bowl, cover, and let rise for 1 hour.
Line a deep 8" bowl with a flour-rubbed dish towel, or generously flour a banneton. Deflate and fold the dough over on itself, then shape it into a ball. Place it, seam-side up, in the prepared bowl or banneton. Cover and let rise until doubled (1 1/2 hours).
Preheat the oven to 425°F; if you have a baking stone, place it on a shelf in the bottom third of the oven.
Invert the loaf onto a piece of parchment on the back of a baking sheet or peel. Spray the top with water, slash the dough, then slide it into the oven.
Bake the bread for 20 minutes, then reduce the oven's temperature to 375°F and bake for another 30 minutes, until the center of the bread reads 205°F when measured with a digital thermometer; the top is a deep golden brown, and the bottom of the loaf sounds hollow when you tap it. Remove the bread from the oven and cool on a rack before slicing.
Tips from our Bakers
- This dough makes excellent sandwich or dinner rolls. For a sandwich roll, divide the dough into 16 pieces. For dinner rolls, divide into 24 pieces. To finish, place on a parchment-lined baking sheet for their last rise (30 to 35 minutes). Bake in a preheated 375°F oven for 20 to 25 minutes for small rolls and 25 to 30 minutes for sandwich rolls.
- What is "ripe" sourdough starter? It's a starter that's been fed and allowed to work at room temperature until at its peak level of activity. It should be full of bubbles; if you watch it for 10 to 15 seconds, you should see bubbles coming to the surface and popping, as if a slow-motion boil is happening.
- This loaf lets time do the work of developing the dough's structure, as opposed to kneading. Too much pushing the dough around will collapse the onions in it, releasing their liquid and making the dough wet and sticky. Gently kneading or folding the dough helps the gluten develop as it rests.
Looking for tips, techniques, and all kinds of great information about sourdough baking? Find what you need in our sourdough baking guide.