Lower heat under same pan to medium and add a drizzle of oil and diced onion. Cook, stirring, until softened, 2-3 minutes. Stir in Southwest spice and diced jalapeño (to taste). Cook until fragrant, about 30 seconds. Add beef, breaking it up into pieces with a wooden spoon. Cook, tossing occasionally, until browned and cooked through, 4-5 minutes. Season with plenty of salt and pepper.