Use a good cheese for the sauce. There are only 3 total ingredients so the flavour of the cheese really stands out. If you want to add a little heat to your sandwich, dice a jalapeño and throw it in with the sauce. Makes 4 sandwiches.

1. To make the cheese sauce:
Whisk the milk and flour in a small saucepan until the flour is completely dissolved. Set over medium-high heat and bring to a simmer, whisking constantly. Reduce the heat to medium-low and continue cooking, still whisking, until the mixture has thickened, about 2 min. Add the cheese and 1/4 tsp salt and stir until the cheese has melted. Simmer the sauce for another 2 min, or until it is thick and velvety. Remove the pan from the heat and press a piece of foil against the surface to keep warm and prevent a skin from forming.

2. Add 1 tbsp of the oil to a large nonstick skillet set over medium-high heat. Season the steak with 1/4 tsp salt and 1/4 tsp pepper. When the oil shimmers, add the steak and cook until just barely cooked through, about 4 min. Remove the meat to a plate and cover with foil.

3. Add the remaining tbsp of oil to the pan (still set over medium-high heat), and throw in the onion and bell peppers. Cook, stirring occasionally, until the onion is tender and the edges of the peppers have browned, about 12-14 min. Season with the remaining 1/4 tsp salt and 1/4 tsp pepper.

4. Meanwhile, preheat the broiler. Once hot, place the halved rolls on a baking sheet and toast them under the broiler.

5. To assemble each sandwich:
Top each halved roll with 1/4 of the onion-pepper mixture and 1/4 of the meat. Drizzle with the cheese sauce (if the sauce has cooled, you can gently reheat it over low heat to re-warm).

TIPS:
In order to slice the steak thinly, I find it helpful to briefly freeze it first (just pop into freezer for about 30-40 min.)