A citrusy sauce that needs little prep time brightens the taste of seared lamb chops for a refined weeknight meal.
Let lamb stand at room temperature for 30 minutes. Pat dry with paper towels. Season with salt and pepper.
Cut ends off 1 orange. Cut off peel, pith, and outer membrane, following the curve of the fruit. Cut between membranes to remove segments, and place in a small bowl. Squeeze juice from membranes into another bowl, and discard membranes. Repeat with pummelo and Oroblancos, combining and reserving 3 tablespoons juice.
Melt butter in a large skillet over medium-high heat. Add lamb, and cook until internal temperature registers 130 degrees on an instant-read thermometer, about 3 minutes per side for medium-rare. Transfer lamb to a platter, reserving pan with drippings. Cover lamb loosely with foil, and let stand.
Add garlic and shallot to drippings in pan. Cook over medium-low heat until softened, about 4 minutes. Juice remaining 2 oranges (you should have 2/3 cup juice). Add juice and sugar to pan, and increase heat to medium. Cook, whisking often, until reduced by half, about 5 minutes. Whisk in creme fraiche, and cook until sauce coats the back of a spoon, about 5 minutes.
Whisk reserved mixed citrus juice and oil in a large bowl. Toss with greens, then season with salt and pepper. Divide salad and citrus among 4 plates. Serve each with 2 lamb chops. Spoon sauce over lamb. Serve immediately.