Cook the spaghetti squash. You can bake it, microwave it, or do as I did and pressure cook it. To pressure cook, cut the squash in half and scoop out the seeds. Place a steamer basket in your pressure cooker, add 1 cup of water, and place the squash halves in the basket. Seal pressure cooker and cook at high pressure for 8 minutes. Release pressure, uncover, and allow the squash to cool until you can handle it comfortably.
Using a fork, scrape out the strands of “spaghetti.” It may take a little effort and going “against the grain.” Put the squash strands into a colander set over a large bowl and set aside. (You can cook the spaghetti squash ahead of time as long as you drain it well before using.)
Heat a medium-sized sauce pan. Add the mushrooms and 1 clove chopped garlic, along with 1 tablespoon of water. Stir and cover tightly. Cook, stirring every 60 seconds, until the mushrooms soften and exude their juices, about 3 minutes. Uncover and cook until most of the liquid evaporates. Season lightly with salt and pepper, if you like, and set aside.
Make the filling: Start your food processor and drop in the 2 cloves of peeled garlic. Process until finely chopped. Add the remaining filling ingredients and process until smooth, stopping to scrape down the sides of the bowl if necessary. Set aside.
Make the cheese sauce: Place all ingredients into a blender and blend until completely smooth.
Preheat oven to 375F. Lightly oil a 2 1/2 to 3 quart oblong casserole dish. (Mine is 9.5 X 6.5 X 3 inches deep.)
Assemble the lasagna: Spoon about 1/4 cup of the spaghetti sauce into the bottom of the dish, just enough to coat the bottom. Spread half of the spaghetti squash over the sauce, and sprinkle with salt and pepper if you like. Drop the filling over the squash by large spoonfuls and then spread it out evenly. Arrange the mushrooms over the filling, followed by the spinach, if you’re using it. Pour half of the cheese sauce over the spinach and mushrooms. Add the remaining squash, smoothing it into an even layer. Pour the remaining spaghetti sauce over the top.
Bake for 15 minutes. Then carefully, starting in the center of the pan, pour the remaining cheese sauce over the top, being careful not to splash the sauce onto the hot pan. Return to the oven and cook until the lasagna is bubbling all around the edges, about 30-40 more minutes. Remove from oven and allow to stand for at least 15 minutes–the sauce will thicken and the lasagna will be less watery the longer it stands. Serve warm.
Since spaghetti squash doesn’t have the starch of lasagna noodles to absorb and thicken the sauce, some liquid in the bottom of the pan is normal. But the lasagna will get firmer the longer it stands uncovered, so you can make it ahead of time and reheat it before serving.