How to Make It
Heat oven to 400° F. Toss the potatoes, sliced fennel, oil, paprika, and 1/2 teaspoon each salt and pepper on a large rimmed baking sheet. Roast until the potatoes are browned on the bottom, 18 to 22 minutes. Turn the vegetables.
Season the cod with 1/2 teaspoon salt and 1/4 teaspoon pepper. Place on top of the vegetables, along with the tomatoes. Continue to roast until the cod is cooked through and the potatoes are tender, 8 to 10 minutes.
Meanwhile, combine the mayonnaise and lemon juice in a small bowl.
Serve the cod and vegetables over the mayonnaise, topped with the fennel fronds.