Place the oil in a very large skillet and set over medium heat. Add the mushrooms and cook until very tender. Season with salt and pepper; remove from the heat. Combine the pasta sauce and stock in a bowl.
Preheat the oven to 350 degrees F. Spread 1/2 cup of the pasta sauce mixture into the bottom of a 9" by 13" pan. Lay 4 to 5 lasagna noodles side by side over the sauce. Spread the noodles with 1/4 of the remaining sauce; top and layer with 1/3 of the spinach. Top the spinach with 1/3 of the mushrooms and 1/4 of the cheese. Layer the remaining noodles, sauce, spinach, mushrooms and cheese in this fashion until all are used. The final layer of noodles will be topped with sauce and cheese only, however, you can decorate it with a few mushrooms and spinach leaves if desired. Cover and bake for 45 minutes. Uncover and bake 10 to 15 minutes more, or until golden and bubbly. Rest the lasagna 10 minutes before slicing.
Options: For an even richer taste, instead of just mozzarella, use a mix of grated Italian cheeses such as provolone, asiago and mozzarella. If you like a spicy flavour, add 1/2 tsp. of crushed chili flakes to the pasta sauce.