Easy French Onion Soup

In a large pot over medium-high heat, melt butter. Add onions and cook, stirring occasionally, until deeply golden, 25 minutes. Add flour and cook 1 minute until no longer raw. Season with salt and pepper, then add white wine and let simmer until evaporated, 3 minutes. Add chicken and beef stocks and thyme and bring to a boil. Reduce heat to medium and simmer 15 minutes. Season with salt and pepper and remove thyme. Preheat broiler to high. Place 2 baguette slices on a large baking sheet and top each slice with 2 tablespoons cheese. Place under broiler until cheese is bubbling and golden brown, 1 minute. Serve soup in bowls or large mugs with baguette slice on top. Garnish with a thyme spring.