1Preheat a grill or grill pan over medium-high heat.
Season the sirloin steak with salt and pepper on both sides.
Grill the steak for 2-3 minutes per side for medium-rare, or until cooked to your desired doneness. Remove from heat and let it rest for 5 minutes.
While the steak is resting, slice the onions thinly. In a small pan over medium heat, sauté the onions until soft and lightly caramelized, about 5-7 minutes.
Toast the 4 slices of bread until golden and crisp.
Spread 10g of pesto on each slice of toasted bread.
Slice the rested steak thinly against the grain.
Arrange the steak slices evenly over the pesto-topped bread.
Top each open sandwich with sautéed onions and sprinkle 10g of grated Red Leicester cheese on each.
Place the sandwiches under the grill for 1-2 minutes, or until the cheese is melted and bubbly.
Serve immediately and enjoy your Steak & Pesto Open Sandwich.