1/3 cup butter or margarine 4 large yellow onions, thinly sliced 1 tablespoon flour 1 1/2 teaspoon salt 4 cups water 1 can (1 pound, 12 ounce) tomatoes, undrained OR 4 cups Italian plum tomatoes, in chunks 6 slices toasted French bread, 1" thick 2 cups grated Gruyere, Swiss or cheddar cheese
Saute onion in Dutch oven with butter, stirring until golden, 15 minutes.
Add flour and cook until it turns golden. Add salt and the water. Bring to a boil; reduce heat, and simmer, covered, 10 minutes.
Meanwhile, put tomatoes through blender to make a puree. Add to onion; simmer, covered, 15 minutes.
Pour soup into individual heat proof bowls or into a large, shallow one. Arrange toast on top; sprinkle with cheese.
Run under broiler 3 to 5 minutes, or until cheese melts and brown slightly. (If your bowls are not broiler safe, a hot oven will do the trick also.)