Heat oven to 400° F. In a food processor, pulse the bread, rosemary, and 1 tablespoon of the oil until coarse crumbs form.
Place the fish on a baking sheet. Spread the mustard on top and sprinkle with the bread crumbs. Bake until the fish is opaque throughout, 10 to 12 minutes.
Meanwhile, heat the remaining oil in a large skillet over medium-high heat. Add the leeks, garlic, and 1/2 cup water. Cook until tender and the liquid has mostly evaporated, 4 to 5 minutes.
Stir in the tomatoes, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Serve with the fish.