Clean spinach. Slice and chop the mushrooms coarsely.
Melt 1/2 of the butter in a skillet. Add 1/2 of the onion, 1/2 garlic, 1/2 mushrooms. Cook for 2 minutes. Add spinach, 1/2 of the salt and pepper. Cook over high heat for 5 minutes to remove all moisture. Set aside to cool.
Split chicken breasts in halves lengthwise. Pound lightly to flatten. Spoon an equal amount of filling down the center of each chicken breast. Fold to enclose filling. Secure with string or toothpicks.
Brown chickens in remaining butter. Add remaining ingredients to skillet and simmer for 15 minutes.
Remove the chicken. Discard toothpicks. Reduce sauce to thicken. Serve over chicken.