Peel and thinly slice onions. Heat 3 tbsp/45 mL butter in a heavy 3-quart/liter saucepan over low heat; add onions and cook slowly. Stir until golden in color, about 10 minutes. Gradually add in water, soup bases, and seasonings. Bring to a boil; cover and simmer about 15 minutes. Meanwhile, toast one side of the bread, under oven broiler, until golden brown. Butter untoasted side and sprinkle slices with 1/4 cup/60 mL cheese; broil until cheese is melted. To serve; spoon hot soup into oven-proof bowls or mugs; float French bread slices on top and sprinkle with remaining cheese, broil until cheese is bubbly and brown.