Mist tortillas with cooking spray; place on a baking sheet. Bake in preheated 400°F oven, turning once, until lightly crisp and golden, about 6 minutes. Set aside.
Meanwhile, finely slice white parts of onions; set aside. Finely slice green parts of the onions; set aside. In a large skillet, cook beef over medium-high heat, breaking up with spoon, until no longer pink, about 5 minutes. Drain off any fat. Stir in white parts of onions, plus garlic, chili powder and vinegar, and salt and pepper to taste; cook for 1 minute. Stir in tomato juice; cook, stirring often, until liquid is reduced by half and sauce is thickened, about 3 minutes.
Meanwhile, in a small bowl, stir together yogurt, lime juice and green parts of onions.
Place a handful of lettuce on each plate. Top with 1 toasted tortilla (or 'tostada'), and top each with an equal portion of remaining lettuce, then beef mixture, tomatoes and cilantro. Finish with yogurt mixture.
Per serving: 287 calories, 27 g protein, 8 g fat (3 g saturated fat), 26 g carbohydrates, 5 g fibre, 59 mg cholesterol, 304 mg sodium.