Heat oven to 425°F. Toss carrots, parsnips, potatoes and onion with 2 tablespoons oil, half the thyme, half the salt and half the pepper. Spread in a single layer on a large rimmed baking sheet; use two sheets if crowded.

Roast 25–30 minutes, turning once, until the roots are nearly tender and lightly browned. If still firm, give them more time before adding fish.

Pat thawed cod dry and check for bones. Rub with remaining oil, thyme, salt and pepper. Make spaces among the vegetables and add fillets without overlapping. Bake 12–15 minutes, checking earlier for thin pieces, until each fillet reaches 145°F in the thickest part and the vegetables are tender.

Remove cooked fish to a clean plate if vegetables need longer. Finish fish and vegetables with lemon juice and divide into four servings. Refrigerate leftovers in shallow containers within 2 hours, or 1 hour above 90°F; keep at 40°F or below, use within 3–4 days and reheat throughout to 165°F.