Preheat the oven to 375 degrees F (191 degrees C). Line a baking sheet with parchment paper, foil, or a silicone mat.

Remove stems from mushroom caps. Bake face up on a silicone lined baking sheet until softened (about 15-20 minutes).

Meanwhile, cook sausage in a skillet on medium-high heat, stirring frequently, until browned (about 7-10 minutes).

Reduce heat to medium. Add garlic and saute for one minute, until fragrant. Add marinara sauce, basil, sea salt, and black pepper. (Adjust seasonings to taste depending on the marinara sauce you use.) Cook for another 2-3 minutes, until heated through.

Set the oven to the broil setting. Drain or blot extra fluid from mushroom caps. Fill mushroom caps with sausage marinara mixture. Top with shredded cheese. Broil on the top rack until cheese is melted and slightly browned (about 3 minutes).