The lamb is cut into three sections for quicker cooking.
Preheat oven to 425 degrees. Scatter lemons in bottom of a roasting pan. Season lamb with kosher salt and pepper. Heat 1 tablespoon oil in a large cast-iron skillet over medium-high. Working in 3 batches, cook lamb until browned, about 5 minutes per side, removing as done and placing on top of lemons in roasting pan. Brush lamb with 6 tablespoons ramp pesto and roast until a thermometer inserted into thickest parts of flesh registers 130 degrees for medium-rare, 20 to 25 minutes. Transfer lamb and lemons to a plate, tented with foil; let stand 20 minutes. Pour pan juices into a small saucepan; keep warm.
Meanwhile, toss asparagus and onions with remaining 2 tablespoons oil; season with kosher salt and pepper. Divide between two rimmed baking sheets and roast, tossing halfway through, until tender and lightly charred, 20 to 25 minutes.
Transfer lamb, lemons, asparagus, and onions to a serving platter. Add any accumulated juices from lamb to saucepan with reserved juices. Brush lamb with remaining 2 tablespoons ramp pesto, pour some of reserved juices over it to moisten, and sprinkle with chopped dill and chives. Season with flaky salt, garnish with chives and mint, and serve.