Warm the olive oil in a large Dutch oven over medium-low heat, and sauté the onion until it starts to soften, about 5 minutes.
Once the onion is soft, add in the garlic and mushrooms, and sauté for another 5 minutes. Add in the butternut squash noodles, spinach, 2 tablespoons of water, and salt and pepper and then cover the pot to let the vegetables cook until tender, about 10 minutes. Stir occasionally to make sure the noodles aren't sticking to the bottom of the pan.
Once tender, add additional salt and pepper if needed and serve warm.