Put 600 ml vegetable stock, some salt and millet in a pot, bring to the boil, cover and cook over low heat for about 30 minutes (do not stir). In the meantime, cut a lid off the peppers at the base of the stalk.
Cut out the seeds, wash and drain the peppers. Clean and wash spring onions and cut into fine rings. Clean and slice the mushrooms. Heat the fat in a pot, fry the mushrooms and finally fry the spring onions briefly.
Season with salt and pepper and remove about half of the mushrooms from the pan. Pour the remaining stock into the pan with the mushrooms and bring to the boil. Mix cream and flour, stir into the pan and simmer for another 1-2 minutes.
Cut half the cress from the bed, stir into the sauce and season with salt and pepper. Loosen millet with 2 forks, mix in mushrooms and 60 g Gruyère and fill into the peppers.
Sprinkle the rest of the cheese onto the filling. Put the mushroom sauce and peppers in a greased, ovenproof dish. Bake in the preheated oven (electric oven: 200 °C/ gas: level 3) for 45-50 minutes.
Cut the remaining cress from the bed and serve the peppers sprinkled with it.