Rinse lemon in hot water, wipe dry and finely grate about 2 teaspoons of the zest. Cut lemon in half, squeeze the grated half and cut the other half into slices for garnish.
Melt the butter in a small pot over medium heat. Stir in mustard, lemon juice and zest.
Rinse salmon steaks, pat dry and coat both sides with lemon butter.
Season with salt and plenty of mixed peppercorns. Cook on a heated grill pan for 5-7 minutes on each side. Serve with lemon slices.