French Onion Soup with Portobello Cheese Toast

Name: French Onion Soup with Portobello Cheese Toast
Ingredients: 4 tablespoons extra-virgin olive oil, divided; 2 tablespoons butter; 2 large sweet onions, sliced; 1 medium red onion, sliced; 4 medium portobello mushrooms, stems and gills removed; ½ teaspoon ground pepper, divided; 2 tablespoons chopped garlic; 2 teaspoons chopped fresh thyme or 1/2 teaspoon dried; ¼ cup dry sherry; ½ teaspoon salt; 4 cups unsalted beef broth; ½ cup shredded Gruyère cheese; ¼ cup plain dry breadcrumbs; 1 tablespoon minced fresh chives