Bring a saucepan over medium heat, and add the rinsed quinoa with water. Once it starts boiling, turn heat to low and simmer for 15 minutes until the quinoa absorbed the water.
Heat a skillet and add olive oil. Once hot, add the sliced mushrooms. Cook for 4-5 minutes. Then, add the garlic, and cook for a minute until fragrant. Reserve.
Spoon the quinoa mixture over the squash halves evenly, top each with the crispy mushrooms and return to the oven just to heat filling for 5-10 minutes.