Season the lamb all over with salt and pepper, then rub with the ras el hanout. Put the potatoes in a medium saucepan and cover with water by 1 inch; season with salt. Bring to a boil, then reduce to a simmer and cook until very tender, 12 to 15 minutes. Drain well.
Combine the butter, lemon zest, all but 2 tablespoons scallions, the finely chopped dill and the chopped cilantro in a small bowl; stir to combine. Add to the drained potatoes and mash with a potato masher or large fork. Slowly add the milk, stirring to combine. Season with salt and pepper and cover to keep warm.
Preheat a large skillet over medium-high heat. Add the vegetable oil and heat until shimmering. Add the lamb chops and cook until well browned, about 4 minutes per side for medium rare to medium.
Divide the lamb chops and potatoes among plates. Top with the remaining scallions, roughly chopped dill and cilantro leaves. Serve with harissa.