Mix butter, garlic, parsley, and red pepper in a small bowl until smooth; season with salt and pepper.

Cover and refrigerate garlic butter for up to 1 day or bring to room temperature before using.

Melt 2 tablespoons of garlic butter in a skillet over medium-high heat.

Add mushrooms and saute until tender, about 3 minutes.

Add spinach and saute until wilted, about 1 minute.

Transfer the mushroom and spinach mixture to a bowl and cool for 30 minutes.

Stir in ground almonds and sour cream; season with salt and pepper.

Loosen the skin of each chicken breast by running your fingers between the skin and meat.

Spread 2 teaspoons of garlic butter under the skin of each chicken breast.

Spoon 2 tablespoons of stuffing under the skin of each chicken breast.

Secure the skin with toothpicks.

Arrange the chicken breasts on a baking sheet.

Spread remaining garlic butter over the top of each chicken breast.

Preheat oven to 350°F (175°C).

Bake for approximately 30 minutes, or until chicken is cooked through.

Transfer chicken to a platter and serve.