In a medium bowl, mix together crayfish meat, spinach, mushrooms, onions, ricotta, Parmesan, oregano, salt, pepper, and garlic.
Slice a horizontal hole in the chicken breast to form a pouch. Spoon 1/3 cup of crayfish mixture into each pouch.
In a large skillet, melt butter over medium-high heat. Arrange chicken in skillet and cook 4 minutes per side or until surface is golden. Move skillet to oven and bake for 35 minutes or until chicken is cooked through.