Preheat oven to 450F. Heat 1 Tbsp. oil in a large skillet over medium-high heat. Add the jalapeno and half the onion. Cook, stirring, until soft, sbout 3 minutes. Add the chicken, 1 tsp. salt and 3/4 cup water. Bring to a boil, then cover and cook until the chicken is tender, about 6 minutes.