Remove the chicken from its packaging and place on a large cutting board. Pat the chicken dry with paper towels, and place breast-side down on the board with the neck facing towards you. Using good-quality kitchen scissors or poultry shears and holding the neck in one hand, cut along one side of the chicken spine, separating it from the ribs. Cut closely to the spine so that you do not remove any excess meat. Continue to cut about ¾ along the spine on one side, repeat on the other side of the spine. Hold the spine for leverage and continue cutting on both sides to remove it completely. If you are having difficulty, rotate the bird so that the tail faces you and cut from the opposite side.
Place the chicken in a marinade container and add the onion, garlic, lemon, rosemary, mustard, paprika, olive oil and salt and pepper. Leave in fridge covered to marinate for at least 1 hour or up to 24 hours.
Place chicken skin side down on a hot grill and place a clean brick or flat rock on top. Grill for 5 minutes and flip chicken. Once both sides are nicely charred, finish cooking the bird on indirect heat. Chicken has to reach internal temperature of 165F. Set aside to rest for at least 20 minutes. Cut in 4 or 8 pieces.
Add the potatoes, garlic cloves, rosemary and salt to a pot and cover with cold water. Bring to a simmer and cook potatoes until you can easily insert the tip of a knife. Strain and transfer into a mixing bowl. Add shishito peppers, drizzle olive oil and season with salt and pepper. Place onto a grill basket and cook until nicely charred.
Place the grilled peppers and potatoes onto a plate leaving room in the middle for the chicken. Garnish with more olive oil and the grilled lemon.