Preheat oven to 400 F. On a baking half sheet lined with parchment paper, place mushrooms stem side up. Season with salt. Bake mushrooms until softened and filled with liquid, about 15 minutes.

Carefully lift each mushroom with a spatula and invert over a bowl to drain out liquid, then place back on the baking sheet. Discard liquid.

Stir together bacon, scallions, cheddar, parmesan, and garlic in a clean bowl until well-mixed. Spoon filling into each mushroom until stuffed.

Bake stuffed mushrooms at 400 F until cooked through and cheese is melted, about 10 minutes.

Let mushrooms cool on the baking sheet for at least 10 minutes, then serve.