In a medium-sized pot, melt butter over medium-high heat.
Peel and quarter the onion; then slice it thinly.
Place onion slices in melted butter. Add a pinch of salt and sauté, stirring occasionally until caramelized, about 8-10 minutes.
Add beef broth, black pepper, and bay leaf.
Bring to a boil; reduce heat to low and cook for 10 minutes.
Meanwhile, heat the broiler to high and toast both sides of the French bread. When toasted, set aside.
Remove the bay leaf and ladle the soup into ovenproof crocks.
Place the bread on top of the soup and top each with 1½ slices of cheese.
Place crocks on a baking sheet; broil for 3-4 minutes until cheese is bubbly and begins to lightly brown. Remove and serve.