Melt butter in a small pan over moderate heat. Add onions and garlic and season with sugar, thyme and pepper. Cook 15 minutes, stirring occasionally, until onions are golden in color and translucent.
Place the split bread on a baking sheet and broil until golden. Spread half the onions and garlic on each half of bread in a thin layer. Sprinkle each side of loaf with a handful of cheese and return to broiler for 30 seconds to bubble cheese. Serve with a hearty, soup-er supper