Prepare the dressing. Whisk together balsamic vinaigrette ingredients. Set aside.
Prepare the chicken. Bring 2 cups water to a boil. Add brine mix and stir until dissolved. Pour over ice in a flat bottomed resealable container. Add chicken, then add additional water as needed until chicken is submerged. Brine for at least 5 hours or up to 12 hours.
Remove chicken from brine and rinse with cold water. Pat dry on a paper towel, then pound down the thicker end of the chicken breasts until they are even thickness across.
Rub the chicken with 1 tsp olive oil until evenly coated. Season with salt, pepper, chili powder, and Oh My Spice! seasoning.
Grill the chicken. Turn your grill on to medium high-heat (7 or 8 out of 10). Allow it to pre-heat until the temperature reads 400°F. Add chicken breast, grilling 6 minutes on one side before flipping. Grill another 5-6 minutes or until a thermometer in the thickest part of the breast reads 160°F (they will continue cooking to 165° after removing from grill). Set aside to rest at least 5 minutes. Note: Depending on your grill, you may need to turn the flames down to medium heat (5 or 6 out of 10) after flipping - you want to get back up to about 400° without overshooting by much.
Grill the peaches. Turn heat on the grill back up to medium high (7 or 8 out of 10). Brush any chicken debris off the grill grates. Spray peaches with coconut oil, then place on the grill cut side down. Grill for about 3-4 minutes, rotating 90° halfway through to acheive the classic grill marks. Remove and set aside, slicing if desired before serving.
Build the salad. Arrange salad with arugula, tomatoes, sliced peaches, torn prosciutto, grilled chicken, and pumpkin seeds. Top with goat cheese, if desired. Pour vinaigrette over salad just before serving.