Make sponge: In a small bowl stir together yeast and warm water and let stand 5 minutes, or until creamy and you can tell the yeast is doing something. In a bowl stir together yeast mixture, room-temperature water, and flour until mixed. Cover bowl with plastic wrap. Let sponge stand at cool room temperature at least 12 hours and up to 1 full day.
Make bread: In a small bowl stir together yeast and milk and let stand 5 minutes. In bowl of a standing electric mixer fitted with dough hook blend together milk mixture, sponge, water, oil, and flour at low speed until flour is just moistened and beat dough at medium speed 3 minutes. Add salt and beat 4 minutes more. Scrape dough into an oiled bowl and cover with plastic wrap. Let dough rise at room temperature until doubled in bulk, about 1 1/2 hours.
Have 2 12- by 8-inch sheets parchment paper ready. Divide dough in half and put each half on a sheet of parchment, forming into an irregular oval about 9 inches long. Dimple loaves with floured fingers and dust tops with flour. Cover loaves with a dampened kitchen towel. Let loaves rise at room temperature until almost doubled in bulk, 1 1/2 to 2 hours.
At least 45 minutes before baking, put a baking stone on oven rack in lowest position in oven and preheat oven to 425° F.
Using the parchment as a sling, place each loaf on your baking stone and bake loaves for 20 minutes, or until pale golden. Let cool 15 minutes before slicing.