Step 1
Preheat oven to 375 degrees F (190 degrees C).
Step 2
Season lamb generously with salt and black pepper. Heat oil over medium-high heat in an oven-proof skillet. Sear lamb on all sides until well browned 3 to 5 minutes per side. Turn off heat. Remove lamb to a platter.
Step 3
Mix together Dijon mustard rosemary cinnamon and cumin in a small bowl. Spread a thin layer of the mustard mixture on each rack of lamb.
Step 4
Place chopped shallots and pinch of salt into the same skillet. Cook and stir over medium-heat until shallots begin to lose moisture and become browned and soft about 5 minutes. Add blueberries and continue cooking for about 2 minutes. Stir in red wine turn off heat and stir mixture to deglaze the pan.
Step 5
Place the racks of lamb into the skillet and interlace the rib bones together to push the lamb racks close together leaving a lamb bone \zipper\ in the center. Place the lamb racks into the center of the skillet. Season with a pinch of black pepper.
Step 6
Place skillet in pre-heated oven and roast lamb until internal temperature reaches 125 degrees F (52 degrees C) for medium-rare about 20 minutes.
Step 7
Remove lamb to a cutting board to rest about 5 minutes.
Step 8
Drizzle balsamic vinegar into the blueberry sauce; add a pinch of salt. Turn heat to medium-high and bring to a simmer to reduce sauce by half about 4 minutes. Turn off heat and stir in butter. When butter is melted check seasonings and add more salt if needed.
Step 9
Slice racks into pieces of 2 ribs each. Serve topped with a spoonful of the blueberry sauce.