Toss diced zucchini, yellow squash, and red onion with 2 tablespoons olive oil, fresh thyme, salt, and pepper on a sheet pan, then roast at 425°F for 8 minutes.
While vegetables start cooking, pat cod fillets dry and rub with remaining olive oil, lemon zest, salt, and pepper.
Push roasted vegetables to the sides of the pan, place cod fillets in the center, and roast for 6-7 minutes until fish is opaque and flakes easily.
Squeeze fresh lemon juice over the entire pan and serve immediately.
Photo: Aurela Redenica / Unsplash