Heat a large saucepan over medium high heat. When hot, add 1 tablespoon avocado oil to the pan along with the onions, a bit of salt, and sauté, stirring often until onions are soft and beginning to brown, about 10 minutes. Remove the onions to a bowl and repeat the process for the mushrooms. When the mushrooms are about done, add the garlic and cook until fragrant. Again, remove the mushrooms to the same bowl as the onions. Cook the asparagus similarly to the other veggies, salting and cooking until tender crisp, another 10 minutes. Combine all vegetables and add the tablespoon of butter, stir well, taste, and season with salt and pepper.