Heat your Blackstone griddle to medium-high (around 400°F). Add vegetable oil and spread it evenly with a spatula.
Add diced steak and onions in a single layer. Season with onion powder, salt, and pepper. Cook for 4–8 minutes, flipping occasionally, until the steak is browned and onions are soft.
Push the steak and onions to one side. Reduce heat to medium on the empty side.
Place tortillas on the cleared area. Add mozzarella and Monterey Jack to one half, then top with steak and onions. Sprinkle more cheese on top.
Fold each tortilla in half. Press gently. Cook 1–2 minutes until golden and crisp underneath.
Flip carefully with a spatula. Cook another 1–2 minutes until both sides are golden and cheese is melted.
Transfer to a cutting board. Rest briefly, slice into wedges, and serve hot.