Trim excess fat and tendons from the chicken breasts and, if needed, trim the underside so pieces are similar thickness; make small crosswise slits in each breast to help them cook evenly and absorb marinade.
In a mortar and pestle or spice grinder, combine dried parsley, dried shallots (or onion), Spike seasoning, dried thyme, garlic powder, and black pepper until evenly mixed.
In a bowl or container, whisk together balsamic vinegar, Dijon mustard, olive oil, and the spice mixture to form the marinade.
Place the chicken breasts in a zip-top bag or snap-top container in a single layer and pour the marinade over them, ensuring all pieces are coated; seal and refrigerate at least 4–6 hours or longer.
When ready to cook, remove chicken from the fridge and drain in a colander to discard excess marinade; allow chicken to come to room temperature while you heat the grill.
Oil the grill grates with a paper towel dipped in oil or use nonstick grilling spray; preheat the grill or grill pan to medium-high heat (hot enough you can only hold your hand above it for a few seconds).
Place chicken on the hot grill diagonally across the grates, cook until grill marks form (about 3 minutes), then rotate each piece to create crosshatch marks and cook a few minutes more.
Flip the chicken and cook the second side until browned and the meat feels firm but not hard when pressed, a few more minutes depending on thickness; remove from heat when cooked through.
Transfer chicken to a plate or cutting board and let rest briefly before serving.