Preheat oven to 375 F and line a baking sheet with parchment paper.
Toss bell pepper (1 thinly sliced + deseeded) and red onions (1/2 thinly sliced) in cooking spray and baking in a preheated oven and until softened to your liking (I cook them for 15 minutes).
Lay roast beef (8 slices) in a single layer on a baking sheet and evenly distribute cheese over the top.
Bake in the oven until cheese just melts, about 5 minutes.
Allow meat/cheese to cool for about 5 minutes (until you're able to handle them), then work quickly before the cheese hardens -- place veggies on one end of the roast beef and roll up over the veggies.
Slice roll-ups in half or enjoy whole. Option to place a toothpick in the center, and enjoy!
Notes
Nutritional information is for 1 roll up if you slice each roll ups in half: