Place a large skillet over medium-high heat with 2 turns of the pan EVOO, about 2 tablespoons. Once hot, add the ground sirloin and cook for about 4-5 minutes, breaking it up with a potato masher or the back of a spoon as it browns.
Add the onion, red bell pepper, garlic, chipotle pepper and sauce, and cook, stirring every now and then for 3-4 minutes, until the onions and peppers start to get a little tender.
In a small mixing bowl, combine the brown sugar, Worcestershire, tomato sauce. Add the sauce to the skillet and season everything with a little salt and pepper, to taste. Bring up to a bubble and simmer until nice and thick, about 3-4 minutes.
To serve, divide the mixture between the split rolls and top each with a few slices of avocado.