1. Heat a large griddle pan over a high heat and brush with vegetable oil. Add the peppers and onions, drizzle with the Worcestershire sauce and cook for 3-4 minutes until lightly browned. Remove to a plate and set aside. Return the pan to a very high heat.
2. Brush the steak on both sides with oil and season with salt and black pepper.
3. Preheat the grill to a high heat.
4. Once the pan is very hot, add the steak and cook for 3-4 minutes on each side for medium, or to your liking. Remove to a plate and allow to rest for five minutes.
5. Lightly toast the buns under the grill until pale golden, then add the Gouda to the top bun halves and place back under the grill until the cheese has melted.
6. Slice the steak against the grain into 1cm thick slices. Toss the slices in the resting juices on the plate.
7. Spread mayonnaise on the bottom bun halves and add some lettuce. Top with the sliced steak and the vegetables and sandwich together with the top halves.

Note: Bavette can be used as an alternative to flank steak, if you wish.

Per Serving: 600kcals, 26.9g fat (9.7g saturated), 36.1g carbs, 9.4g sugars, 49.5g protein, 3.3g fibre, 0.607g sodium