Heat olive oil over moderate heat in a large skillet. Stir in onion and garlic, reduce heat to low and cook just until softened. Add the herbs and pepper and cook one additional minute. Transfer mixture to a large bowl. Stir in 1 cup of the water, milk, salt and 2 tsp sugar. In a small bowl, dissolve the yeast and pinch of sugar in remaining 1/2 cup warm water. Let stand until creamy and bubbling. Add to the onion mixture and stir in flour, one cup at a time. When dough is too stiff to stir, turn onto a lightly floured surface and knead in as much flour as required to keep the dough from sticking. Continue to knead until the dough is smooth and elastic.
Generously grease a large bowl with additional oil. Transfer dough to bowl, coat with oil and cover with plastic wrap. Set aside in a warm spot and let rise until doubled (about 2 hours). Punch down the dough, re-cover and let rise again until doubled (about 1 hour).
Brush olive oil on the bottom and sides of a 17 by 13 by 1 inch baking sheet (or jelly roll pan). Sprinkle with cornmeal and tap out excess. Punch down the dough and turn it onto a cornmeal dusted work surface. Coat the dough lightly with cornmeal and shape it into a rectangle. Place the dough in the pan and pat to fill corners. Brush with egg white mixture and sprinkle with a combination of poppy and sesame seeds. Let the dough rise, uncovered, until doubled and puffy (45-60 minutes).
Preheat oven to 450 degrees and position a rack in the bottom third. Dip your fingers in flour and make deep indentations in the dough. Sprinkle with coarse salt, if desired. Bake 30 minutes, or until golden brown. Makes one 17 x 13 inch focaccia.