In a large saucepan, sauté the onions and tomatoes for 5 minutes.
Add the drained, pre-soaked lentils. Pour in the vegetable stock and simmer everything together for 15 minutes.
Add the garlic and continue to cook for 5 minutes.
Blend the soup until smooth and season to taste with salt and pepper.
To serve, with a vegetable peeler, shave the Gruyère AOP on top and garnish with the finely chopped mint.