Turn the heat to medium high, add the tomatoes, capers, caper brine, and lemon zest, give it 2 or 3 good stirs to distribute the ingredients, and let the tomatoes cook until you hear 3 or 4 of them pop. Mash or pierce the tomatoes with a potato masher or a large fork at this point, then add the chicken stock and chili flakes and stir to redistribute the ingredients again.