Reduce heat to medium-low, add ¼ cup oil and cook, stirring often, until oil is absorbed and beans start to dry out at edges, about 5 minutes or so. Continue to cook, adding remaining oil by ¼-cupfuls every 5 minutes and stirring occasionally, until very thick and dark, 15–20 minutes. Discard jalapeño. Season with more salt if needed. Serve with sour cream, avocado, and tortillas.