Set a saute pan over medium-high heat, and add a drizzle of olive oil. Once the oil shimmers, add the garlic and saute until fragrant. Add in the onions and mushrooms and stir once, to incorporate with the garlic. Let them sear for a minute or 2 before stirring, so they get color. Stir once, and leave for another minute, then add in squash and cook until squash is browned but not watery. Remove from heat, and move sauted veg into a container. Place pan back on heat.