Sweet tomatoes get cooked until they break down, creating a jammy sweet-and-sour sauce that’s made even better with bacon. Brushing the fish with a lemon-mustard mixture before cooking is an easy way to boost its flavor.
4 slices thick-cut bacon, chopped
1/2 large red onion, chopped (about 5 oz.)
1 pint cherry or grape tomatoes, halved
1 Tbs. apple cider vinegar
1 Tbs. light brown sugar
2 Tbs. Dijon mustard
1 Tbs. lemon juice
4 6-oz. skinless cod or haddock fillets, at least 1 inch thick
Kosher salt and freshly ground black pepper
Position a rack in the center of the oven and heat the oven to 425°f. Line a large rimmed baking sheet with foil.
Cook the bacon in a 12-inch skillet over medium heat until crisp, 10 to 12 minutes. Using a slotted spoon, transfer the bacon to a paper-towel-lined plate.
Pour off all but 2 Tbs. of the bacon fat from the skillet. Add the onion to the skillet, and cook over medium heat, stirring occasionally, until starting to soften, about 3 minutes. Add the tomatoes, vinegar, and sugar, and cook, stirring occasionally, until the tomatoes break down, about 10 minutes. Stir in the cooked bacon.
Meanwhile, whisk the mustard and lemon juice in a small bowl. Pat the fish dry with paper towels and arrange on the prepared baking sheet. Brush with the mustard-lemon mixture, and season lightly with salt and pepper. Bake the fish until just opaque in the center, 10 to 12 minutes. Spoon some of the tomato-bacon jam onto each plate, top with the fish, and serve.