6 plum tomatoes, halved lengthwise
2 tablespoons olive oil, plus more for baking sheet
3/4 teaspoon dried oregano
pinch of sugar
1/3 cup mayonnaise
1 tablespoon fresh lemon juice, plus lemon wedges for serving
3 slices white sandwich bread, torn into large pieces
4 flounder fillets (1 and 1/2 pounds)
2 tablespoons Dijon mustard
Preheat oven to 450 degrees, with racks in upper and lower thirds. Place tomatoes on a rimmed baking sheet, cut side up. Drizzle with 1 tablespoon oil, and sprinkle with 1/4 teaspoon oregano and sugar; season with salt and pepper. Roast on lower rack until tender, 20 to 25 minutes.
Meanwhile, make sauce: In a small bowl, whisk together mayonnaise, remaining 1/2 teaspoon oregano, and the lemon juice; season with salt and pepper.
Lightly oil another rimmed baking sheet. In a food processor, pulse bread with a pinch each of salt and pepper and remaining tablespoon oil until fine crumbs form. Pat fish dry with a paper towel, and season both sides with salt and pepper. Place fillets on oiled baking sheet. Dividing evenly, spread mustard on top; sprinkle with bread crumbs, gently pressing to adhere. Bake on upper rack until fish is cooked through and coating is browned, 6 to 8 minutes. Serve with roasted tomatoes, sauce, and lemon wedges alongside.