Heat butter in Dutch oven over medium-high heat. Add onions and 1/2 teaspoon salt; stir to coat with butter. Cook over medium-low heat about 35 minutes until onions are golden brown, stirring often.
Add broth, red wine and bay leaf. Simmer gently for 15 minutes. Remove bay leaf. Add remaining 1/4 teaspoon salt and the pepper. Ladle equally into four 1-1/2-cup ovenproof bowls. Add a slice of toast to each; push down to saturate with broth.
Mix cheeses and thyme. Completely cover bread and soup with cheese mixture, dividing equally. Place on baking sheet. Bake at 425° F about 10 minutes, until cheeses melt and turn lightly brown. Serve immediately.