In a large pot or Dutch oven, melt a bit of oil over medium-low heat. Add sliced yellow onions and cook them slowly for about 30 minutes until they are caramelized.

Once the onions are beautifully caramelized, add your drained butter beans and pour in the vegetable broth, stirring well. Bring to a gentle simmer for about 10 minutes.

Taste and season the soup with salt and freshly cracked black pepper, then let it simmer for another 10-15 minutes.

Preheat your oven to 400°F (200°C). Slice the baguette and arrange the slices on a baking sheet. Toast the bread in the oven for about 5-7 minutes.

Ladle the hot soup into oven-safe bowls and sprinkle grated Gruyère cheese over the top of each serving.

Place the bowls under the broiler for 3-5 minutes until the cheese is bubbly and golden brown.

Remove the bowls from the oven, and optionally drizzle with olive oil and sprinkle with fresh herbs before serving.