Restaurant-Style French Onion Soup

INGREDIENTS

• 1/2 cup butter
• 8 onions, sliced
• 2 cloves garlic, crushed
• 1/2 teaspoon salt, or to taste
• 1/2 teaspoon ground black pepper, or to taste
• 1 teaspoon dried thyme
• 1 cup red wine
• 1 tablespoon sherry
• 2 tablespoons all-purpose flour
• 2 quarts beef broth
• 1 French baguette, cut into 1/2-inch thick slices
• olive oil, or as needed
• 6 slices Swiss cheese
• 6 slices provolone cheese