Steak Sandwich

Method

1. Pat the 150 g sirloin dry and season it generously on both sides with salt and coarse black pepper. Leave it on the board for 20 minutes so it comes fully to room temperature — a cold steak greys before it crusts.
2. Stir the 1 tbsp of mayonnaise together with the 1 tsp of Dijon mustard and a good grind of pepper. Set aside.
3. Heat 2 tsp of the olive oil in a heavy cast-iron or steel pan over medium-low heat. Add the 0.5 onion, thinly sliced, with a pinch of salt and cook 12 minutes, stirring now and then, until limp, sweet and deep golden at the edges. Scrape onto a plate.