Preheat oven 400F degrees.
Butterfly the chicken breasts: slice almost all the way through the thickness of each breast, stopping when you can open the breast like a book and lay the attached halves flat.
Pound each breast lightly. Set aside while you make the filling.
Filling: Heat the olive oil in a large skillet. Add onions and saute over medium-hight heat for about 2 minutes. Stir in mushrooms, garlic, salt, and pepper. Cook, covered, for about 3 minutes. Remove the lid and continue cooking until liquid is absorbed. Add cilantro and transfer mixture to a plate to cool.
When the filling is cooled, divide it into four equal portions. Mound one portion in the center of each chicken breast. Fold all the edges of the breast meat over the filling to enclose it completely.
Coating: In a bowl, combine the bread crumbs, oil, cilantro, salt, and pepper.
Place the milk in another bowl.
Dip the stuffed chicken breasts in the milk and then roll them in the seasoned bread crumbs.
Arrange the coated breasts in a roasting pan and bake for 20-25 minutes, or until cooked through but still moist.