1. Whisk the ground almonds, flour, and zest together and season with salt and pepper.
2. Pat the fish fillets dry. Set up the fish, the egg yolk, and the ground almonds assembly line fashion.
3. Using a pastry brush, lightly coat one side of each fillet with a little of the beaten egg yolk. (I coat the side that would have had skin.) Dip the coated side of each fillet into the nut mixture.
4. Place a large nonstick skillet over medium heat. Add 1 tablespoon of the butter and a small pinch of salt, if your butter isn’t salted, and cook the butter until it turns light brown, about 3 minutes. Slip the fillets into the skillet, nut side down, without crowding the skillet. Reduce the heat and cook until the coating is golden and the fish is cooked halfway through, 3 minutes or so. Season the exposed side of each fillet with salt and pepper, add another 1/2 tablespoon cold butter to the pan, and very gently turn the fillets. Cook, spooning some of the browned butter over the fillets once or twice, until the fish is opaque throughout, about 2 minutes more. If it looks like the pan is dry, add a little more butter.
5. Give each flounder fillet a squirt of lemon juice, then scatter over some toasted almonds and parsley. Have more lemon wedges at the table so you can give the fish another squeeze or two if needed.