Preheat oven to 350°F (175°C).

In a skillet, combine ground sausage, diced white onion, crumbled dried sage, garlic salt, and black pepper.

Cook over medium heat until sausage is browned, breaking it apart with a spoon.

Remove skillet from heat and drain any excess grease, if necessary.

Add shredded pepper jack cheese to the sausage mixture and mix well until the cheese starts to melt and bind the sausage together.

Spoon the sausage and cheese mixture into the cleaned mushroom caps.

Place the stuffed mushroom caps in a baking dish.

Bake in the preheated oven for 15 minutes, or until the mushrooms are tender and the cheese is melted and bubbly.

Serve hot and enjoy!