1️⃣ Melt butter in a large pot over medium-high heat. Add onions and cook for 10 minutes until softened.
2️⃣ Reduce heat to medium, add salt and sugar, and cook for 35 minutes, stirring every few minutes until onions are deep golden brown and syrupy.
3️⃣ Stir in garlic and cook for 1 minute until fragrant.
4️⃣ Pour in red wine to deglaze the pot, scraping up all the flavorful bits.
5️⃣ Add broths, Worcestershire sauce, thyme, and bay leaf. Simmer for 20 minutes, then remove herbs.
6️⃣ Stir in balsamic vinegar, season with salt & pepper, and keep warm on low heat.
7️⃣ Preheat broiler and arrange French bread slices on a baking sheet. Brush with olive oil and broil for 2-3 minutes, flipping once until toasted.
8️⃣ Ladle hot soup into oven-safe bowls, top with toasted bread, and sprinkle generously with Gruyère, Mozzarella, and Parmesan.
9️⃣ Broil until cheese is bubbly and golden brown. Serve immediately!