Sprinkle steak with 1/4 tsp (1 mL) each salt and pepper. In a large skillet, melt butter over medium-high heat; brown steak on both sides, about 2 min per side. Transfer to plate.
Reduce heat to medium; add garlic, red, yellow and green peppers and thyme to skillet. Cook, stirring often, for about 5 min or until soft. Whisk flour into milk; whisk in mustard and Worcestershire sauce. Gradually stir milk mixture into skillet, scraping up any brown bits stuck to pan.
Simmer, stirring often, for about 5 minutes or until slightly thickened. Return steak and accumulated juices to pan, spooning sauce over steak. Simmer, turning steak once, for 5 to 10 min or until steak is desired doneness. Transfer steak to cutting board; let rest for 5 min. Season sauce to taste with salt and pepper. Slice steak thinly across the grain and serve with sauce.
