Cook cremini mushrooms in butter over medium-high heat for 5-6 minutes until all moisture has evaporated, stirring as they dry out and turn fragrant with visible browning. Add garlic and fresh thyme leaves and cook 1 minute, just until the garlic smells toasty, then remove from heat.

Cool the mushroom mixture slightly so the cheese won’t seize, then stir in shredded mozzarella cheese and Parmesan cheese until fully combined and glossy.

Cut a deep pocket in each boneless skinless chicken breasts, keeping the sides intact so the filling stays inside.

Season each chicken breast inside the pocket and on the outside generously with salt, pepper, garlic powder, and smoked paprika so the surface will caramelize.

Fill each pocket with the mushroom-cheese mixture and secure the opening with toothpicks.

Sear the stuffed chicken in olive oil in an oven-safe cast iron skillet over medium-high heat for 3-4 minutes per side until deeply golden with a caramelized crust.

Bake at 400°F for 18-22 minutes until the internal temperature reaches 165°F, with the chicken set and the filling steaming at the edges when sliced.

Remove toothpicks, drizzle with extra garlic butter, and rest for 5 minutes before slicing so juices redistribute and the cheese stays molten.