Spaghetti Squash Lasagna

Ingredients -- Ovo-Lacto Vegeterian (optional meat)
Here's a keto-ish lasagna that avoids semolina flour pasta. The idea is to use thin-sliced keto vegetables instead. Cooked spaghetti squash can be part of the "filling". Cooked spaghetti squash is a good "natural" keto substitute for pasta.
Many store-bought pasta alternatives are available today. "Palmini" imitation pasta is made from hearts of palm and is healthy, but very stiff and fibrous. It's a good alternative for lasagna sheets, but should be boiled thoroughly to soften before assembling the lasagna, and then cooked again along with the lasagna.
To make your own lasagna sheets, you will need an 8"x8" baking dish and a "Mandolenne" (device that slices large vegetables very thin). Be careful, those things are sharp.

If meat is desired, pan-fry 1 lb. ground Italian sausage until mostly cooked.
1 large Zucchini and/or 1 small Eggplant. "Palmiri" lasagna noodles made from hearts of palm can substitute here.
1/2 of a cooked Spaghetti Squash
4 oz diced Mushrooms
1/2 white Onion, diced fine
2 cans fire-roasted Tomatoes
handful of Spinach leaves, washed, stems removed, roughly chopped
Olive oil for various purposes
Seasonings to taste: salt, black pepper, oregano, basil, generous amounts of garlic

Peel the zucchini and/or the eggplant. Slice diagonally with a Mandolenne to get long, thin slices. Place on a baking sheet covered in parchment paper, and place in the oven at 350 degrees F for 10 minutes. This helps to dry them out a bit.
While your zucchini/eggplant is baking, place your sausage (if used) and the diced onions in a large skillet with a little olive oil over med-high heat. Cook until sausage & onions are browned, then add in your mushrooms and cook a couple more minutes.
By this time your zucchini should be done and you can start layering the pan. First add a little olive oil to coat the bottom and sides to ensure nothing sticks. Then place half your zucchini and/or eggplant "noodles" on the bottom of your 8×8 baking dish.
Drain the canned tomatoes very well so that no water remains. In a separate bowl, place the spaghetti squash along with the tomatoes, and stir vigorously until they mush together somewhat and the tomatoes aren't intact. Add the seasonings here. If it seems like too thick a mush for you to stir & spread easily, add additional canned tomatoes or tomato sauce.
Spread half your meat and mushroom mixture over the zucchini/eggplant "noodles" in the baking dish, then half the tomato mixture. Spread the tomato layer evenly, then top with a thin layer of spinach. (You can also add a layer of cheese on top of the spinach if you want extra cheese, but I just add it to the top)
Then add another layer of the rest of the zucchini, then the meat/veggie mix, then top with the other half of the squash & tomatoes, spinach leaves and cheese on top.
Bake at 450 degrees F for about 15-20 minutes, or until cheese is browned and zucchini noodles are cooked through. Let sit for 10 minutes before cutting and serving.