Trim any fat from steak; slice steak into thin strips. In bowl, toss steak with chili powder and 1/4 teaspoon (1 mL) each of the salt and pepper.
In large nonstick skillet, heat 1 tablespoon (15 mL) of the oil over medium-high heat; cook steak, in batches and stirring, for 2 1/2 minutes or until browned. Remove to plate.
Add remaining oil to skillet; cook onion, half of the garlic, the cumin, hot pepper flakes and 1/4 teaspoon (5 mL) each of the salt and pepper, stirring, for 3 minutes.
Add sweet peppers and zucchini; cook, stirring, for 3 minutes or until tender-crisp. Return meat to skillet; add mustard, and coriander (if using) and toss gently.
Meanwhile, wrap tortillas in foil; heat in 350ºF (180ºC) oven for 5 minutes or until warmed through.
Peel and pit avocados; mash with lime juice and remaining garlic, salt and pepper.
Spread 2 tablespoons (25 mL) avocado mixture over tortillas. Divide beef mixture among tortillas and roll up. Serve with remaining avocado mixture.