Preheat an oven to 425 degrees F. Slice the spaghetti squash in half lengthwise and scoop out the seeds and pulp. Drizzle a small amount of oil over the halves and sprinkle with a pinch of salt. Place the spaghetti squash on a baking sheet cut-side down and bake for 30-45 minutes, until it is tender.
Heat the olive oil in a skillet over medium-high heat. Add the garlic and onion to the pan and saute until the onion is translucent and fragrant, about 3 minutes.
Sprinkle the chicken lightly with salt and pepper and add to the pan. Cook for 3 minutes on each side.
Add the mushrooms to the pan and saute for another 3-5 minutes, until the mushrooms have softened and the chicken is cooked through. Add in the lemon juice and stir until combined. Remove from heat.
Serve the chicken and mushrooms over spaghetti squash, and top with fresh basil. Enjoy!