Heat olive oil and butter in a large skillet. Add onions and cook over medium-low heat for 25–30 minutes until deeply golden and caramelized.
Add garlic and thyme; cook for 1 minute.
Stir in Dijon mustard and vegetable broth. Simmer for 5 minutes.
Add butter beans and gently simmer for 8–10 minutes until creamy and flavorful.
Season with salt and black pepper.
Sprinkle with Gruyère and broil for 2–3 minutes until melted and golden.
Garnish with fresh parsley and serve with crusty bread.
