Add yeast to the warm water and let stand for 10 minutes.
In a large measuring cup or medium-sized saucepan, heat milk and margarine to 135 degrees (about 1 1/2 minutes in the microwave or 4 minutes in a saucepan on medium heat). Pour this liquid into a large mixing bowl. Add sugar, salt, and minced onions. Cool to lukewarm. Add cornmeal, egg, Parmesan cheese, softened yeast, and mix well by hand or with a mixer.
Add flours to make a soft dough. Turn onto a lightly floured surface and knead until smooth and satiny, 10 minutes by hand, or 4 minutes in an electric mixer or food processor fitted with dough hook. Put in greased bowl, turning to coat thoroughly. Cover with damp cloth and allow to rise in a warm, draft-free place until doubled, about 1 hour.
Knead down dough. Turn onto a lightly floured surface. Divide dough in half and let rest for 10 minutes. Shape each piece into a long narrow loaf, about 14 inches long and 1 1/2 inches in diameter. Place on prepared sheet or in baguette pans and sprinkle with cornmeal. Cover with a damp cloth and allow to rise in a warm, draft-free place until doubled, about 45 minutes.
Preheat oven to 350 degrees F. Bake 25 to 30 minutes. Remove from pans and cool on racks.