Bring a medium pot of salted water to a boil, add the asparagus, and blanch until bright green and crisp-tender, 2 to 3 min. Drain and set aside.
In a very large skillet, cook the bacon over medium heat until crisp. Remove (leave the fat in the pan), drain on a paper towel, and set aside with the asparagus.
Add 1 Tbs. of the butter to the skillet, heat until foaming and add about half of the mushrooms. Cook over medium-high heat until browned and the mushrooms have released their juices, 10 to 15 minutes. Increase the heat to high and cook until the mushrooms are dry and nicely browned. Transfer them to a bowl and reserve. Add another 2 Tbs. butter and cook the second half of the mushrooms in the same way. Transfer them from the pan to the other mushrooms.
Reduce the heat to medium high. Add the last 1 Tbs. butter and sauté the shallot, scallions, and garlic for about 2 minutes until softened and lightly browned. Add the sherry and deglaze the pan by scraping up any browned juices. Boil until the liquid is reduced to a syrupy glaze. Add the stock, tomato paste, and thyme. Simmer for a few minutes, and then whisk in the butter paste bit by bit. Simmer another minute until the sauce thickens a bit. Add the cooked asparagus, bacon, and mushrooms to the sauce, taste, and season with salt and pepper. Pour into a 2-qt. baking dish.
Pile the potatoes loosely on the filling for a more appealing, fluffy look.
Heat the oven to 375°F. Drop spoonfuls of the mashed potatoes on the mushroom filling and spread gently to form an even (but not too smooth) layer. Bake in the hot oven until the potatoes are light brown and the filling is bubbling, 25 to 35 minutes.