Whisk together stock, lemon juice, mustard, rosemary, and pepper flakes in a bowl. Season with salt.  Heat a 12-inch cast-iron skillet over medium heat until hot, 1 to 2 minutes. Season chicken with salt and pepper. Place, skin sides down, in the skillet. Place a second smaller skillet on top of chicken and weigh it down with soup cans. Cook until the skins are golden brown and crisp, 7 to 9 minutes.  Remove cans and top skillet. Flip chicken and reduce heat to medium-low. Add garlic and cook until the chicken is cooked through, 2 to 3 minutes. Transfer chicken to a platter; reserve skillet. Add stock mixture to reserved skillet. Cook, scraping up any brown bits, until slightly thickened, 1 to 2 minutes. Spoon over chicken.