Heat 1 tbsp olive oil in a skillet.
add the garlic and onions and simmer until garlic is fragrant, approx. 2 minutes. Stir often to prevent the garlic from browning.
Add the mushrooms and cook until tender, stirring occasionally.
Dump the spaghetti squash strands into the skillet.
Add the pesto, tomato sauce and shredded parm.
Stir to coat the squash.
Season with salt and pepper to taste.
Evenly redistribute the squash mixture into each squash bowl.
If topping with mozzarella, do so, and give the squash broil until mozzarella melts (see notes).
Garnish with fresh basil and serve.