1. Heat oven to 350°F. Line baking sheet with parchment paper.
2. In medium skillet, heat oil over medium heat; cook onion and bell peppers until soft, stirring frequently.
3. Add roast beef and lightly toss with vegetable mixture.
4. Separate dough into 8 triangles. Divide vegetable-beef mixture evenly among triangles; top each with a sprinkle of cheese.
5. Fold dough around filling; seal completely.
6. Place croissants on baking sheet. Bake 12 to 15 minutes or until light golden brown.