To make the marinade:
Add the vinegar and lemon juice to a blender. Replace the lid, turn on the blender, and through the feed opening, add, 1 at a time, the garlic, salt, and black pepper. Leaving the blender running add the olive oil in a slow thin stream.
Place the mushrooms and chicken breasts in separate nonreactive bowls suitable for marinating and pour half of the marinade mixture over the contents of each container. Add the sliced red onion to the container with the chicken. Cover and marinate in the refrigerator for at least 2 hours.
Begin making the warm potato salad. Boil potatoes until tender but not mushy, about 10 minutes. Meanwhile, fry the bacon and drain on paper towels. Add the balsamic vinegar to a blender, replace the lid, and through the feed opening, add, 1 at a time, the dill, mustard, honey, pepper, and salt. Leaving the blender running, add the olive oil in a slow thin stream. Drain the potatoes well, add the scallions, crumble in the bacon, and fold in enough of the dressing to coat.
Heat the grill and grill marinated Portobello, skin side down first for about 5 minutes over indirect heat. Flip and grill the other side. Remove from heat and let rest.
Grill both sides of marinated chicken, about 5 minutes per side, and transfer to a baking sheet.
Preheat oven to 425 degrees F.
In a small bowl, mash together the butter, breadcrumbs, sage, parsley, thyme and salt and pepper, to taste. Chop grilled mushrooms finely and combine with butter/breadcrumb mixture to make a paste. Press onto surface of chicken breasts and finish in oven until crust turns golden brown, about 2 to 5 minutes.
Arrange warm potato salad and crusted chicken breast on plate and sprinkle with parsley.