Lightly toast 2 slices of bread until golden brown and crisp.
In a small bowl, mix 4 tbsp mayonnaise, 1 tsp Dijon mustard, 1/4 tsp ground black pepper and a small pinch of garlic powder (optional) until smooth.
Spread the mayonnaise mixture evenly over both slices of toasted bread.
If using cheddar, place 2 slices of mature cheddar on one slice of bread.
Top with 120g thinly sliced gherkins, arranging them evenly.
Place the other slice of bread on top, press gently, then cut the sandwich in half. Serve immediately.