1. Make the cranberry-mustard glaze:
preheat a medium nonstick skillet to medium-high heat. Add in the fresh cranberries, unsalted butter, granulated sugar, and 1 tsp kosher salt. Stir to combine and let the sauce come to a simmer. Continue to cook the sauce until the cranberries have started to burst using a wooden spoon to smash the cranberries down. Let the sauce cook for about 10 to 12 minutes until slightly thickened. Remove from the heat and let cool before stirring in the whole grain mustard. Set aside.

2. Preheat the oven to 325 degrees:
Spray a 10.5 x 7.5  baking dish with non-stick cooking spray. Place the sliced onions in the bottom of the baking dish and toss with olive oil, salt and pepper. 

3. Take the American lamb and place it on top of the onions:
Cook the lamb for 35 minutes. Flip the lamb and cover it with cranberry-mustard sauce, and continue to cook for an additional 35 minutes, or until the internal temperature reaches 145 degrees for medium rare (roughly 20 to 25 minutes per lb). Remove from the oven and allow to rest for three minutes before slicing and serving with labneh.