To make the dough, in a large bowl, combine the yeast, salt, water, sugar, and 5 cups of the flour. Knead the dough on a floured surface, adding flour until the dough is elastic but not too dry. Let rise in an oiled bowl until doubled in bulk, about 50 minutes. (The dough can be left to rise in the refrigerator overnight.) Knead again and divide into 2 oblong loaves. Sprinkle an oiled baking sheet with cornmeal. Place the loaves on the sheet. Let rise until doubled in bulk, about 50 minutes.
Preheat the oven to 350 degrees F. To make the stuffing, melt the butter in a large skillet. Add the onion and saute until tender. Add the spinach, pepper, dill weed, and salt. Add the cheese and cook until melted. Using a sharp knife, make a large slash in the top of each loaf and place the stuffing inside. Close the slash by pinching the dough. Brush the dough with the egg and sprinkle with sesame or poppy seeds (if using). Bake for 30 to 40 minutes, or until golden brown.