Heat onion and garlic in olive oil until the onions are softened up. Add the sausage for browning. Drain the oil.
Stir in diced tomatoes, sauce, paste, mushrooms, and spinach. Add more or less sauce to your opinion. I like to have plenty of sauce. Stir well.
Mix in Italian seasoning, crushed red pepper, sugar, black pepper, and salt. Let sauce simmer over med-low heat while you do the next step.
For cheese filling – combine ricotta, beaten egg, parmesan, and about 1 cup of the mozzarella.
Layering the lasagna. Start with a thin layer of sauce on the bottom of your pan. Place your noodles. Spread half the ricotta mixture on the noodles. Ladle a thick layer of sauce on top of the ricotta mixture. Sprinkle a thin layer of mozzarella over the sauce. Place the last layer of noodles. Spread the rest of the ricotta mixture. Pour the rest of the sauce. Sprinkle the rest of the mozzarella.
Bake in oven at 375 degrees for 30-35 minutes.
I use a pretty deep lasagna pan and mine goes all the way to the top. You may want to put an old cookie sheet under the pan in case you have any bubble over while in the oven.