1. To prepare soup, heat oil in a large Dutch oven over medium-high heat. Add onion to pan; sauté 15 minutes or until almost tender, stirring frequently. Reduce heat to medium-low; cook until deep golden brown (about 40 minutes), stirring occasionally.
2. Increase heat to medium. Add mushrooms to pan; cook 10 minutes or until mushrooms are tender, stirring frequently. Stir in garlic and thyme sprigs; cook 2 minutes, stirring frequently. Increase heat to medium-high. Add wine to pan; cook 2 minutes or until most of the liquid evaporates. Add broths to pan; bring to a simmer. Reduce heat, and simmer 45 minutes. Stir in salt and pepper. Discard thyme sprigs.
3. To prepare toasts, preheat broiler.
4. Arrange bread in a single layer on a baking sheet. Top each bread slice with 1 teaspoon Gruyère and 1 teaspoon Gorgonzola. Broil 2 minutes or until cheese melts. Sprinkle chopped thyme over cheese. Ladle about 1 cup soup into each of 6 bowls; top each serving with 2 toasts.