01 - Preheat oven to 375°F. Line a baking sheet with parchment paper or lightly grease it.
02 - Place mushroom caps, cavity-side up, on the prepared baking sheet. Set aside.
03 - Heat olive oil in a skillet over medium heat. Add ground turkey and cook, breaking up with a spoon, until no longer pink, about 4 to 5 minutes.
04 - Add minced garlic and cook for 1 minute until fragrant.
05 - Stir in chopped spinach, salt, and pepper. Sauté until spinach is wilted, 2 to 3 minutes. Remove from heat.
06 - Mix in basil pesto and grated Parmesan cheese until evenly combined.
07 - Spoon the filling into each mushroom cap, pressing gently to fill.
08 - Sprinkle shredded mozzarella over the tops for extra meltiness, if desired.
09 - Bake for 18 to 22 minutes, until mushrooms are tender and cheese is golden and bubbly.
10 - Let cool slightly before serving.