In a large nonreactive saucepan, heat the oil. Add the onion and garlic and cook over moderately low heat until barely tender. Add the chicken and cook over moderate heat until it is no longer pink, about 3 minutes. Stir in the chili powder, cumin and oregano. Add the stock, tomatoes with their juice and jalapeños. Season with salt and pepper. Cover, bring to a boil and simmer over low heat for 15 minutes.