Preheat broiler in oven and set rack about 3-4 inches below heating elements. 
Cut all visible fat from sirloin steak. In a small bowl, combine chili powder, oregano, and cumin. Rub the spice mixture over both sides of the beef and let stand for 15 minutes at room temperature. 
Heat broiler; broil steak approximately 3 to 4 minutes per side or until done to your preference using a meat thermometer (see below). 

Rare - 120°F 
Medium Rare - 125°F 
Medium - 130°F

When the steaks are done to your liking, remove from oven, let sit 15 minutes before serving (meat temperature will rise 5 to 10 degrees after it is removed from the oven). Slice steak across grain into 1/8 inch slices. 
Warm tortillas according to package directions. 
In a small bowl, combine the sour cream and horseradish. Spread 3 tablespoons of the sour cream mixture over the top of each tortilla. Top each tortilla with 1 cup of lettuce; sprinkle with chopped red onion and tomatoes. Divide beef into equal portions and top each tortilla with beef. 
Fold one end of a tortilla up about 1 inch over filling then fold the left and right sides over the folded end, overlapping and serve. 
Makes 6 servings.

Ingredient Options:
Substitute cooked and shredded chicken.
Substitute chopped fresh or roasted red, green, yellow, or orange bell peppers.

Each Serving Totals - 12.3 Fat Grams, 339.3 calories, 7.9 WW Points