Heat oil in a large skillet over medium heat.
Add frozen sliced franks and sauté for 3 minutes, stirring continuously.
Add garlic and stir for an extra minute.
Mix in onions, tomato paste, mustard paste, salt and stir to combine flavors.
Add bell pepper and mix together. Let simmer.
Mix in black pepper and set aside.
Split the sub rolls in half, lengthwise. Hollow out the bread from the bottom side of each roll.
Layer bottom of the bread with lettuce.
Add cooked filling and serve hot.