2 medium leeks, white and light-green parts only, thinly sliced, rinsed well, and patted dry
1 teaspoon freshly grated lemon zest
2 tablespoons fresh lemon juice
1 tablespoon olive oil
3 sprigs fresh thyme
2 and 1/2 cups cherry tomatoes (about 12 ounces)
4 cod fillets
Preheat oven to 450 degrees. In a 9-by-13-inch baking dish, toss together the leeks, lemon zest, lemon juice, oil, thyme, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Cover with foil, and bake until the leeks just begin to soften, 8 to 10 minutes.
Remove the baking dish from the oven. Add the tomatoes, and toss to combine. Season both sides of cod with salt and pepper; place on top of vegetables.
Cover the dish and bake until fish is opaque throughout, 15 to 20 minutes. Serve immediately.