1. Boil pasta and make the béchamel sauce (recipe below).
2. Preheat the oven to 400F and place your bread slices in there. Bake them while you're cooking everything else, until they are toasted through.
3. Saute onion in olive oil, add tomatoes and cook for 6-8 min, stirring often. Season with salt and pepper and set aside.
4. Melt margarine in a pan and add mushrooms. Cook until the juices have been released. Add garlic (and a little lemon juice if desired), season, and cook until most of the juices have evaporated.
5. When bread is done, remove from the oven and out in a food processor or finely chop to make breadcrumbs.
6. To assemble, start with a little béchamel sauce, then a layer of pasta. Layer on half of the tomato mixture, and another thin layer of béchamel. Add another layer of pasta, then half of the mushroom mixture, and béchamel. Repeat with the rest of the tomato and mushroom mixtures. Finish with a layer of pasta and béchamel.
7. Add about 2 Tablespoon of olive oil to the breadcrumbs and mix with your hands so it's nicely coated. Sprinkle on top. Bake for 20 minutes and serve.