Bring the milk to the boil, sprinkle in the semolina and prepare according to the instructions on the packet. Then let it cool down.
Mix with quark after cooling. Then add the apple pieces, nuts, cinnamon, agave syrup and - if you like - the grated lemon peel. Separate the eggs. Beat the egg white until stiff and fold into the semolina mass together with the egg yolk. Put everything in a buttered baking dish.
Put in the hot oven and bake for about 50 minutes. If everything went well, the casserole has come up nicely and looks slightly brown.