Preheat the oven to 425°F. Line one half-sheet pan with a double layer of paper towels and another half-sheet pan with non-stick, extra-wide, heavy-duty aluminum foil or parchment paper. Set both pans aside until ready to use.
Add Kosher salt, reserving one teaspoon to use later, to a large pot of water and to boil. Add the Brussels Sprouts, and once the water returns to boil, cook for eight minutes until bright green and tender. While the sprouts are cooking, prepare an ice bath.
Drain the sprouts in a colander and immediately add to the prepared ice bath to stop them from cooking.
Once all the ice has melted, the sprouts should be cooled. Transfer them to the paper towel-lined baking sheet to dry. Place more paper towels on top to help absorb some of the water.
Dry the bowl used for the ice bath. Then, add the minced garlic, thyme, black pepper, the remaining teaspoon of Kosher salt, and olive oil. Whisk everything in the bowl together. Add the dried Brussels sprouts and stir to coat them in the oil mixture.
Spread the Brussels sprouts on the foil-lined baking pan. Smash them flat with the bottom of a glass or jar. Sprinkle the tops of the Brussels sprouts with freshly grated Parmesan and shredded mozzarella cheese.
Bake in 425°F oven for 25 minutes, or until the cheese is melted and golden. Sprinkle with freshly minced parsley and serve warm.
Yield 6 servings.