Cook for about 7-10 minutes, until mushrooms are browned and have released their liquid.
Heat the butter in a saucepan until melted and add the flour, whisking constantly for 2-3 minutes.
Add the milk, a little at a time, whisking constantly, until a sauce is formed.
Remove from heat and stir in the nutmeg, salt and pepper.
Place a layer of bechamel sauce in the bottom of a 8×13″ baking dish. Add a layer of noodles.
Add the sliced tomatoes and half of the mushrooms.
Add dollops of ricotta cheese over the mixture and sprinkle with Romano.
Cover with another layer of noodles and spread a light layer of bechamel sauce over the top.
Add the second half of the mushrooms and dollop with the ricotta cheese, then sprinkle with Romano.
Top with the remaining bechamel sauce and sprinkle with the Mozzarella cheese.
Cover with aluminum foil and bake in the oven for 45 minutes. Remove foil and bake an additional 15 minutes uncovered.