To make the brine, sauté the garlic and onion in oil until translucent, 3-4 minutes. Add the kosher salt after the onion and garlic have cooked for 30 seconds or so. Add the rosemary and cook to heat it, 30 seconds. Add the water and lemon, squeezing the juice into the water. Bring the water to a simmer, stirring to dissolve the salt. Remove from the heat and allow the brine to cool. Refrigerate until chilled.

Place all the chicken pieces in a large, sturdy plastic bag. Set the bag in a large bowl for support. Pour the cooked brine and aromatics into the bag. Seal the bag so that you can remove as much air as possible and chicken is submerged in the brine. Refrigerate for at least 8 and up to 24 hours, agitating the bag occasionally to redistribute the chicken.

Remove the chicken from the brine, rinse under cold water, pat dry, and set on a rack or on paper towels. Ideally, the chicken should be refrigerated, uncovered, to dry out the skin, but it can be cooked at once.

Combine the flour, black pepper, paprika, salt, cayenne, and baking powder in a bowl. Whisk to distribute the ingredients. Divide this mixture between two bowl. Pour the buttermilk into a third bowl. Set a rack on a baking sheet.

Dredge the chicken in the seasoned flour, shake off the excess, and set the dusted pieces on the rack. Dip the pieces in the buttermilk, then dredge them aggressively in the second bowl of seasoned flour and return them to the rack.

Heat the oil in a large, high-sided pot for deep-frying to 350ºF. Add as many pieces of chicken as you can without crowding the pan. Cook the chicken, turning the pieces occasionally, until they are golden brown and cooked through, 12 to 15 minutes depending on their size. Remove to a clean rack and allow them to rest for 5 to 10 minutes before serving. Fry the rosemary sprigs. When ready to serve, garnish with lemon zest and fried rosemary sprigs.