Meanwhile, prepare the sauce: Melt butter over medium heat-high heat in a sauce pot. Add flour, chipotle and adobo sauce, stir to combine. Add heavy cream slowly, stirring well with each addition to incorporate. Add cheese and stir until melted. Keep over low heat while finishing sandwich.
Remove flank steak from freezer and slice on the bias against the grain into 1/4-inch thick pieces. Heat 1 tablespoon of oil in a cast iron griddle or skillet, and saute the red peppers until they blacken around the edges, and soften, about 8 minutes. Remove from griddle and turn heat to medium-high, add more oil if necessary and sear steak in batches, about 2 minutes.
To assemble the sandwiches: Cut bread loaves in half, leaving 1 side attached. Layer on avocado slices, top with flank steak, red pepper, and cilantro. Drizzle with chipotle cheese sauce and serve immediately.