Heat oil in a large skillet over medium-high heat. Add onion and jalapeno; season with salt and pepper. Cook, stirring occasionally, until softened, 4 to 5 minutes.
Add garlic, chili powder, and cumin to skillet; stir until fragrant, about 1 minute. Add beef; cook, breaking up meat with a spoon, until no longer pink, about 5 minutes. Add tomatoes; simmer over medium-low heat, stirring occasionally, until thickened, 10 to 12 minutes. Season with salt and pepper, as desired.
Meanwhile, warm tortillas in a dry skillet over medium heat or directly over a gas burner, about 30 seconds on each side. At the table, fill tortillas with beef mixture and desired garnishes.