In 25 cm skillet, melt butter until sizzling; add onions and peppers. Cook over medium-high heat, stirring occasionally, until onions are golden (6 to 8 minutes). Remove from heat. Stir in garlic salt, cheese and beef strips.
Place about a sixth of the filling in center of each warm tortilla. Fold two opposite edges of tortilla toward center over filling. Roll up open end of tortilla toward opposite edge. Place, seam-side down, on microwave-safe plate.
Microwave two sandwiches on high, turning or rearranging after half the time, until heated through (1 to 2 minutes). Repeat with remaining sandwiches.