Program for sauté. Add butter. Once butter is melted, add onions. Cook onions until they are softened and browned, about 10 minutes.
Add vegetable broth, bay leaves, and thyme. Program for manual-high. Set time for 10 minutes. Allow for NPR (natural pressure release) for 10 minutes, or completely.
Remove bay leaves. Add soup to individual oven-safe bowls. Lay a slice of bread on top of the soup, then cover with cheese. Broil 2 minutes, watching closely. You want the cheese to melt and brown, but not burn.