Preheat oven to 375°F (190°C) and line a baking sheet with parchment paper.
Heat olive oil in a skillet over medium-high heat; sauté onions and bell peppers until softened and slightly browned, approximately 5 minutes. Season with salt and pepper.
Unroll crescent dough and separate into 8 triangles.
Place a portion of chopped roast beef on the wide end of each dough triangle.
Top the beef with a spoonful of the sautéed vegetable mixture and one half-slice of provolone cheese.
Roll the dough from the wide end toward the point, ensuring the filling is tucked in, and place on the baking sheet.
Combine melted butter and garlic powder; brush the mixture over the top of each crescent roll.
Bake for 12 to 15 minutes until the pastry is golden brown.
Remove from oven and let rest for 2 minutes before serving.