Preheat oven to 400º F. Spray a large baking dish with cooking spray and set aside.
Add olive oil to a large skillet and heat over medium-high. Add the onions and carrots and cook until onions are translucent, about 5 minutes. Add the mushrooms and continue to cook until all vegetables are tender, about 10 minutes more. Add the asparagus and garlic and stir to combine. Remove vegetables to a bowl.
Prepare the mashed potatoes according to the package directions.
Heat skillet again and add the ground beef and brown completely. Discard all fat and return to stove top. Add the beer and cook on high until reduced by half, about 5 minutes. In a small bowl or glass measure, add the cold water and cornstarch and stir to combine. Add to meat mixture and cook on high until the sauce thickens and starts to bubble. Turn off the heat and add the vegetables and stir to combine. Pour into prepared baking dish and top with the prepared mashed potatoes. Bake for 40 minutes at 400º F. Remove and serve immediately.
Recipe by Farmgirl Gourmet at https://www.farmgirlgourmet.com/2014/03/easy-stout-shepherds-pie.html