Heat the oven to 350°. Spread the bread out on a baking sheet and bake, tossing occasionally, until deep golden brown, about 25 minutes. Transfer the croutons to a rack and let cool. Meanwhile, finely grate the zest from the lemons into a small bowl. Using a paring knife, cut off the top and bottom of each lemon and remove the white pith. Cut between the membranes of the lemon to release the segments into the bowl with the zest, and then squeeze the juice from the membrane into the bowl; discard the membrane.
In a 12-inch nonstick skillet, heat 4 tablespoons of the butter over medium-high. Place the flour on a plate, and then season the flounder with salt and pepper and dredge it lightly in the flour, knocking off the excess. Add the fillets to the butter and cook, turning once, until golden brown and cooked through, 3 to 4 minutes.
Using a spatula, transfer each fillet to a plate and return the skillet to medium-high heat. Add the remaining 4 tablespoons butter and cook, swirling the skillet, until it begins to smell nutty and turn brown. Stir in the lemon zest, segments, and juice along with the chopped parsley and capers and remove from the heat. Season with salt and pepper and then spoon the lemon sauce over the fillets and top with the croutons and parsley leaves.