Using a pastry brush, coat the chicken breasts with the no-stick marinade. Refrigerate 30 minutes. Sprinkle the breasts with poultry seasoning.
Prepare the grill. Cook chicken over direct high heat for 10-12 minutes, turning once.
While the chicken is cooking: Heat butter in a medium sauté pan over high heat. Place mushrooms, salt, and pepper in the hot butter and sauté the mushrooms for 6-7 minutes, stirring them occasionally. Drain off any excess moisture.
With the chickens on the grill, top each chicken breast with one each of both cheeses. Divide the onions and mushrooms evenly and place them atop the cheese. Close the lid to the grill and cook until cheese melts.
Place chicken on serving dishes and top with the fresh tomatoes and chopped parsley.
Yield: 6 servings