How to Make It
Prick potatoes all over with a fork; place on a plate. Microwave potatoes at high in 3 (5-minute) intervals until tender. Halve potatoes; scoop out potato pulp into a large bowl, leaving a 1/4-inch border around skin; set halves aside. Add milk, 1 tablespoon butter, 3/4 teaspoon salt, and 1/2 teaspoon pepper to potato pulp; mash until smooth. Cover to keep warm.
Heat oil in a medium skillet over medium-high. Add mushrooms and thyme; cook, stirring occasionally, until browned, 6 to 7 minutes. Add broth, vinegar, and Worcestershire; bring to a simmer, and cook until liquid is reduced by half, about 2 minutes. Stir in 1 1/2 teaspoons butter and remaining 1/4 teaspoon pepper. Transfer mushroom mixture to a small bowl; cover to keep warm. Wipe skillet clean.
Return skillet to heat over medium-high. Add garlic and remaining 1 1/2 teaspoons butter; cook, stirring constantly, until garlic is slightly golden, about 30 seconds. Add spinach in batches, letting each batch cook down slightly before adding the next. Cook, tossing constantly with tongs, until liquid has evaporated, about 2 minutes. Stir in remaining 1/4 teaspoon salt. Transfer to bowl with potato pulp; stir to combine.
Divide potato mixture evenly among potato halves. Top evenly with brisket, mushroom mixture, cheese, and onions.