Brown the ground beef in a large pan, drain any excess liquid, then stir in the taco seasoning.
Add a splash of chicken broth and cook the onions until softened.
Arrange 4 tortillas on a baking sheet, overlapping slightly to create one large base.
Spread on the Greek yogurt, then layer the seasoned beef, tostadas, tomatoes, onions, crushed taco shells, and shredded lettuce.
Fold everything toward the center, spray the top lightly with cooking spray, then cover with another baking sheet.
Bake at 400°F (200°C) for 20 to 25 minutes, or until golden brown and crispy on top.
Let cool for a few minutes, slice into 9 pieces, and enjoy.
