Add the remaining 1 teaspoon of olive oil to the skillet along with the shallot and cook over moderate heat until softened. Add the tomatoes and cook, tossing, until heated through, about 2 minutes. Add the vinegar and cook until nearly evaporated, about 30 seconds. Add the stock, season with salt and pepper and simmer until slightly reduced, about 2 minutes. Spoon the tomato sauce over the chicken and serve.