Rub the steak all over with the oil and season, to taste, with salt and freshly ground pepper.
Heat a chargrill pan until hot and fry the steak for 3-4 minutes per side (for medium), or until cooked to your liking. Remove from the pan and set aside to rest on a warm plate. Place the Stilton on top of the steak to melt in the residual heat.
Meanwhile, place the tomato halves, cut-side down, and the bread into the chargrill pan used to cook the steak. Fry for 3-4 minutes, turning the bread halfway through cooking to toast on both sides.
For the salad, place the spinach leaves into a bowl and pour over the oil and lemon juice. Season, to taste, with salt and freshly ground pepper and mix well.
To serve, place a slice of toast onto a serving plate. Top with the steak, tomato halves and spinach leaves. Finish with the remaining slice of toast.