1: Make the marinade for the lamb chops: In a mixing bowl, combine the mustard and garlic. Whisk in the olive oil (about 1/4 to 1/3 cup) and about 1 1/2 teaspoons dried oregano. (Crush the oregano between your palms to release its fragrant oils.) Season with salt and pepper.
2: Pour a few spoonfuls of the marinade in the bottom of a glass baking dish. (Alternatively, marinate the chops in a resealable plastic bag.) Add the chops, then top with the remaining marinade. Turn the chops to coat. Set aside (or refrigerate) for 20 to 30 minutes.
3: In the meantime, set up your grill for grilling in the embers and heat to high.
4: Generously oil the potatoes and season with salt and pepper. Enclose in 2 foil packets in a single layer. Put the packets directly on the coals, turning once or twice with long-handled tongs.
5: When the potatoes are tender (a skewer should go through them easily), set them aside in their foil packets and keep warm.
6: Replace the grill grate. When it’s hot, brush or scrape it clean and oil it well with vegetable oil.
7: Season the tomatoes with salt, pepper, and dried oregano, and arrange, cut sides down, on the grate. Arrange the lemons (also cut sides down) on the grate at the same time. If you have room, arrange the lamb chops on the grate. Otherwise, grill the tomatoes and lemons first. Turn the tomatoes once with a spatula or tongs; you want them to remain rather raw on the inside. Transfer the tomatoes and lemons to a platter large enough to hold them and the lamb chops.
8: Grill the chops for 2 to 3 minutes per side, or until done to your liking. Transfer to a sheet pan topped with a wire rack and let rest for 2 to 3 minutes. In the meantime, remove the potatoes from the foil packets and use the palm of your hand to smash them on a cutting board. Season with salt and pepper. Transfer to a platter and serve with yogurt, if desired.
9: Arrange the lamb chops on the platter with the tomatoes and lemon halves. Top the tomatoes with the feta, kalamata olives, and chopped fresh oregano. Serve immediately.
