Also: A shallow gratin dish, about 19 x 29 cm, well buttered.
Heat the butter/oil in a large saucepan, add the onions, carrots and celery and stir briefly. Put lid on, simmer at medium temperature for 10 min, stirring from time to time. Put lentils in a colander and briefly rinse under cold water, then add to vegetables form and stir everything together well. Add water and salt and pepper. Bring to a boil, put the lid on, lower the temperature so that the mixture simmers gently. Simmer on low heat for 25-30 minutes until the vegetables are soft and the lentils have turned a pale color. Season again, pour the mixture into the prepared gratin dish and spread the cheese and bread crumbs evenly on top. This can be well prepared the day before. Then bake only for 30-40 min. in the oven at 180 °C.
Or if everything is to be prepared and served together in one, the lentil mixture must be steaming hot when it is transferred to the gratin dish. Later just bake under the broiler.