Heat a large skillet over medium-high heat, adding a splash of oil. Toss in the diced onion, stirring for 3–4 minutes until translucent and fragrant.

Add the ground beef to the skillet, breaking it apart as it cooks. Stir for 5–7 minutes until browned and no longer pink.

Sprinkle the taco seasoning over the browned beef and cook for an additional 2–3 minutes.

Reduce heat to low, layer shredded cheddar and Monterey Jack cheeses over the beef, cover, and let it melt for 1–2 minutes.

Slice the hoagie rolls open and toast the cut sides in a separate pan with butter for 2–3 minutes.

Spoon the cheesy beef mixture onto the toasted hoagie rolls, and top with diced tomatoes, cilantro, and jalapeños.

Close the sandwiches and serve immediately while hot.