Clean and dry the 16 ounces white button mushrooms, removing the stems to create caps. Set them aside.
In a small pan, melt 3 tbsp butter and sauté the 1 teaspoon garlic for 1 minute. Add the 2 teaspoon flour whisking for 2 minutes. Gradually pour in the 1 1/3 cups half and half while stirring, then add the 2/3 cup grated Parmesan cheese. Simmer until the sauce thickens and is smooth. Remove from heat and set aside.
In a bowl, mix 1/4 cup breadcrumbs, melted 3 tablespoon butter, 1/4 teaspoon Kosher salt, 1/4 teaspoon black pepper, grated w1.5 cups white cheddar cheese, and 1/4 cup of the Alfredo sauce (reserve the rest for drizzling). Stir until the mixture is well combined.
Spoon the breadcrumb mixture into each mushroom cap, pressing gently to fill them evenly.
Preheat the air fryer to 350°F (175°C). Place the stuffed mushrooms in a single layer in the air fryer basket. Air fry for 8-10 minutes or until the mushrooms are tender and the stuffing is golden brown and crispy on top.
Drizzle with the remaining Alfredo sauce and serve warm.