Preheat oven to 450 degrees.
Toss together cauliflower, onion, capers and 2 tbsp olive oil on a rimmed baking sheet; season with Mrs. Dash Lemon Pepper and fresh ground black pepper to taste.
Spread vegetables in a single layer and roast, stirring with a spatula halfway through, until cauliflower is tender and browned, 30-40 minutes.
Meanwhile, cook pasta in a pot of boiling water until al dente, according to package instructions. Drain.
Toss pasta with roasted veggies, remaining 2 TBS olive oil, parsley, and lemon zest. Toss gently to combine. Garnish with toasted, crumbled walnuts; serve.