Preheat oven to 425F. Toss carrots, parsnips, and red onion with glug of olive oil, salt and pepper. Spread on a sheet pan and roast until tender and caramelized at the edges.
Pat fillets dry and season both sides with salt and pepper. Heat olive oil in a large skillet over medium high heat. Place fish skin side down and cook until skin is crisp. Flip carefully and cook until fish flakes easily with a fork.
In a small saucepan, melt butter over medium heat. Cook until it turns golden brown and smells nutty. Remove from the heat and stir in lemon zest, lemon juice and parsley.
To assemble, spoon roasted vegetables onto plates, top with whitefish and drizzle generously with lemon brown butter.