To make the mustard spread, whisk all ingredients together and refrigerate in a cover jar for up to 2 weeks.
If the chicken is completely thawed, place it on a plastic-wrapped plate and put it back in the freezer for 15-20 minutes. The chicken needs to be slightly frozen so that it’s easier to slice thinly.
Use a very sharp knife and thinly slice the chicken. A food processor fitted with the slicer blade works very quick wonders here!
Season the chicken with a couple dashes of Worcestershire, and salt and pepper.
Heat 1 Tbsp olive oil over medium heat.
Add the peppers and onions, toss to coat with oil, and cook covered for 10-15 minutes, until softened, stirring every few minutes. If necessary, add a couple Tbsp of water to prevent sticking.
Transfer peppers to a bowl and set aside.
Heat remaining olive oil over medium high heat and add the chicken, stirring frequently until cooked through and lightly browned.
Drain fat and reduce heat to medium.
Add peppers back to the pan, tossing to reheat.
Spread mustard on split hoagie rolls and spoon mixture on top.
Top each with a slice of cheese and serve hot. Pop the sandwiches under the broiler, if necessary, to reheat and melt cheese.