In a glass baking dish, combine garlic, oil, vinegar, salt, oregano, pepper and cumin; add steak, turning to coat. Let marinate for 15 minutes.
Guacamole: Meanwhile, peel and pit avocado; mash in a bowl with yogurt until fairly creamy, with just a few chunks remaining. Stir in tomato, cilantro, green onion, salt and hot sauce, if using.
Heat a large skillet over medium-high heat; cook steak, turning once or twice, until medium-rare in centre, 8 to 10 minutes. Transfer to a cutting board and let rest for 5 minutes before slicing thinly crosswise into ½ inch (1 cm) thick slices.
To assemble, spread each tortilla with 2 tbsp (25 mL) of the guacamole; in a line along centre, layer with rice, beans and corn. Top with cheese and olives, if using. Top with sliced steak, then lettuce; fold in ends and wrap tightly. Serve with additional guacamole, if desired.
Per serving: 547 calories, 38 g protein, 18 g fat (4 g saturated fat), 59 g carbohydrates, 10 g fibre, 50 mg cholesterol, 799 mg sodium