In the same saucepan, heat the olive oil. Add the leek, garlic and poblano and cook over low heat, stirring occasionally, until softened, about 12 minutes. Add the beans and their cooking liquid and bring to a simmer. Add the tomato and simmer over moderate heat until tender, about 3 minutes. Stir in the thyme and tarragon and season with salt and pepper. Serve the stew hot or warm.