To make the lemon butter: Melt the butter over low heat in a small saucepan. Add the lemon juice and white wine. Keep the sauce warm on a corner of the stove.
To prepare the fish: Whisk the eggs with the heavy cream in a large bowl. Put the flour in a separate bowl. Dredge the fillets in flour; shake off any excess then put the fillets into the batter.
Heat the butter in large heavy-bottomed skillet over medium heat. Lift the sole out of the batter, allow the excess to run off, then gently lay the fillets in the pan. Cook, turning the fish once, until the sole is flaky and the crust is crisp and golden brown, about 3 minutes per side.
While fish is cooking, toast the bread and cut each piece in half diagonally (corner to corner). Arrange the toast points on 4 plates, 2 per plate. Lift the fish from the pan onto the toast points. Drizzle the fish with lemon butter and serve immediately garnished with a sprig of fresh parsley, if desired.