In a large pot, heat olive oil over medium heat. Add sliced yellow onions and stir to coat with oil. Cook slowly for 25-30 minutes until golden brown and caramelized.

Pour in vegetable broth and add drained butter beans. Stir well, increase heat to medium-high and bring the soup to a gentle simmer. Cook for about 15 minutes.

For a creamier texture, use an immersion blender to blend a portion of the soup directly in the pot for a few seconds while leaving some beans whole.

Ladle soup into oven-safe bowls filling them about three-quarters full. Generously top with grated Gruyère cheese. Preheat your broiler on high and place bowls on a baking sheet.

Place baking sheet under the broiler for about 3-5 minutes until cheese is melted and golden brown.

Remove from oven and let cool slightly. Drizzle with olive oil and sprinkle fresh herbs if desired. Serve immediately with toasted baguette.