To make the tomato sauce: sauté the onion, garlic and mushroom over medium heat in butter and oil for 20 minutes until cooked. The mushrooms should have released their juice, it should have evaporated, and everything should be a lovely golden brown.
In a large pot or crock pot, combine the mushroom mix with the rest of the tomato sauce ingredients: water, lentils, stock cubes, tomatoes, tomato paste, basil, oregano, rosemary, brown sugar, pepper, salt, and red wine.
Simmer at least an hour on the stovetop, or in the oven - more if you have the time. 2-3 hours is nice. I like to leave it overnight in the crock pot or slow cooker to really reduce it to a rich, thick sauce. Remember lentils will probably need a bit longer, and feel free to add more water if they're particularly thirsty.
To make the Bechamel: In a medium saucepan, melt the butter and sauté the flour for at least one minute.
Add the milk, bay leaf, nutmeg and salt and simmer over medium heat, stirring regularly, until sauce comes to a boil and thickens. Remove from heat.
Prepare the lasagna: In a large baking dish, spread a little of the tomato sauce and put a layer of lasagna sheets over the top.
Ladle half of the tomato sauce over, top with a third of the bechamel, and sprinkle 1/2 cup cheese.
Add another layer of pasta, the rest of the tomato sauce, a third of the bechamel, and 1/2 cup cheese.
Add a final layer of lasagna, and top with the remaining bechamel and sprinkle 1 cup cheese over.
Bake 45min - 1 hour until lasagna sheets are soft, all is heated through, and the cheese is golden and bubbling.
Serve and enjoy!