Caramelize the onions

• Heat the olive oil and butter in a frying pan over medium-low heat.
• Add the sliced onions with a pinch of salt and cook slowly, stirring often, until deep golden and jammy.
• Add the garlic and thyme and cook until fragrant.

Cook the lentils

• Bring the vegetable broth to a simmer in a saucepan.
• Add the rinsed lentils and simmer until tender but not mushy.
• Drain, keeping a little of the cooking liquid.

Combine and season

• Stir the caramelized onions into the lentils.
• Add the soy sauce, balsamic vinegar and Dijon mustard.
• Season with salt and pepper and loosen with a splash of cooking liquid if needed.

Broil the topping

• Spoon the lentil mixture into a baking dish.
• Top with toasted baguette slices and grated Gruyère.
• Broil until the cheese is melted and bubbling.

Serve

• Divide the bowls and serve hot.