Bring a small pot of water to a boil. Clean fiddleheads, removing brown fuzz and trimming the ends. Then cook in boiled water for about 10 minutes.Heat 4 cups frying oil to 350 degrees in a deep pot. Whisk together 1 cup flour, 2 Tbsp. cornstarch and a pinch salt. Slowly add 1-1 ½ cup cold seltzer and whisk just until smooth and combined. Spread the bread crumbs out on a plate. Dip the cooked fiddleheads in the batter and then roll in the bread crumbs. Fry them in batches, for about 20-30 seconds on each side, about 1 minute total, until lightly golden. Remove from oil and let them drain on paper towel. Sprinkle with fine salt and serve with lemon wedges.