While squash is roasting, prepare the filling.
Heat 2 tbsp of salted butter or olive oil (if making vegan) in a pan.
Saute chopped onions and minced garlic in butter or olive oil (if making vegan) until the onions are translucent and the raw smell of onion and garlic is gone. Add 1 tsp of salt while cooking onions, it makes this steps a little faster.
Once onions and garlic are cooked, add washed/chopped mushrooms.
Keep sautéing mushroom until they are completely cooked and all the water from mushroom is evaporated, this step takes good amount of time.
Once the mushrooms are cooked, add chopped baby spinach and cook it until the spinach is wilted and all the water from spinach has been evaporated.
Take it off the stove and let it sit for 5 minutes. Then add ricotta cheese or squeezed and crumbled tofu (if making vegan). Add ground black pepper, red chili flakes, garlic powder and dry oregano.
Add 3 tbsp of heavy cream (or soy milk/EVOO mix if making vegan) and mix it very well.