Place the lamb on a half-rimmed baking sheet or roasting pan. Coat it with about 2 tablespoons of pesto, turn it fat side up, and let it marinate in the refrigerator for at least 1 hour, but up to 48 hours.
Preheat oven to 450°F.
On the same baking sheet you're using for the lamb, make a little aluminum foil or parchment paper boat on half of the baking sheet. Add 1 bag of cauliflower rice, 1 tablespoon oil, 3/4 teaspoon salt, 1/2 teaspoon pepper, and 1/2 teaspoon garlic powder. Stir to combine.
Roast for 15 minutes. Remove baking sheet from oven. Stir cauliflower rice. Flip lamb over. Add 1 pound asparagus and toss with 1 tablespoon oil, 1/2 teaspoon salt, and a couple of grinds of pepper.
Roast for 8 to 10 minutes. Check the lamb temperature starting at the 8 minute mark — it should be 145°F internally for medium. I like my asparagus crisp, so I find that it’s perfect after 10 minutes. If you want yours more tender, keep roasting it.
Add 2 tablespoons butter and optionally, a drizzle of milk or heavy cream to the cauliflower. Mash with a fork until it’s the consistency of chunky mashed potatoes. Or: throw it in the blender with a splash of milk to make a cauliflower puree! Adjust seasoning to taste.
Carve the lamb by slicing between the rib bones. Serve the lamb over the cauliflower mash, drizzled with pesto. Serve the asparagus on the side.