Cut mushrooms into 1/4" slices. Trim woody ends off asparagus. Halve and peel onion. Slice halves into thin slices. Peel potatoes and cut into 1" dice. Place potatoes in a medium pot with covered by cold water and 1 tsp. salt.

Bring pot with potatoes to a boil, then reduce heat to simmer. Cook until potatoes are fork-tender, 15-18 minutes. Drain in colander and return to pot. Add half the butter (reserve remaining for asparagus) and half the heavy cream (reserve remaining for gravy). Mash with a fork or potato masher until smooth. Season with 1/2 tsp. salt and a pinch of pepper and set aside. While potatoes simmer, cook asparagus.

Heat a medium non-stick pan over medium-high heat. Add remaining butter and asparagus to hot pan. Cook until asparagus are bright green and tender, 4-5 minutes. Season with a pinch of salt and pepper. Transfer to a plate and wipe pan clean. Reserve pan.

Combine ground beef, liquid egg, half the thyme (reserve remaining for gravy), 1/2 tsp. salt, and 1/2 tsp. pepper in a medium mixing bowl and mix until just incorporated. Form mixture into two 1" thick oval steaks. Heat 2 tsp. olive oil in pan used for asparagus over medium heat. Add steaks to hot pan and cook until browned and steaks reach a minimum internal temperature of 160 degrees, 6-8 minutes per side. Remove to a plate. Discard excess fat, leaving 2 tsp. behind. Reserve pan.

Return pan used to cook steaks to medium-high heat. Add onions to hot pan and cook until slightly caramelized, 4-5 minutes. Add mushrooms and cook until browned, 5-8 minutes. Add sherry and remaining thyme. Cook 1 minute and remove pan from burner. Stir in remaining heavy cream and season with a pinch of salt and pepper.

Place whipped potatoes on plate. Serve Salisbury steak and asparagus next to potatoes. Spoon mushroom gravy over steak and serve.