Preheat the grill to high temperature.
Wash and chop the meat (lamb or any other meat) in pieces. Chop them in sizes that will fit perfectly on the grill.
Mix one tablespoon of oil and season with salt and pepper in a small container; use the mixture to brush the meat chops lightly on both sides. Place on a platter and let it marinate while you prepare the sauce.
Combine the chopped onion, the ¼ cup oil, and the mustard in a small, heavy saucepan and bring to boil; occasionally stirring to distribute the heat evenly and avoid getting the meat burnt; keep the heat low and simmer gently for about 10 minutes until the onion is soft and lightly browned. Stir frequently.
Thin the mixture to sauce consistency with three tablespoons of water, add more if needed. Turn off the heat and taste, add more pepper and salt if you want. You can add sugar too, but this is optional.
Remove from the heat and cover to keep warm.
Oil the grill grate with olive oil. Arrange the marinated meat on the hot grate and grill. Turn the meat pieces with tongs, until it is cooked. (6 to 8 minutes per side for medium)
Transfer the meat to serving plates or a platter and spread the onion mustard sauce on top.
You can also serve with a French baguette and a glass of freshly squeezed fruit juice.
