Preheat oven to 350 degrees F. Prick potatoes all over with a fork. Place the potatoes on a baking tray and bake for 30 to 45 minutes, until knifetender. (If you like, this can be done the night before and potatoes can be refrigerated until ready to fry.) Cut each potato into 6 or 8 wedges, depending on the size. Pour about 4 inches oil into a large heavy pot and heat to 375 degrees F. Fry half of the potato wedges at a time for 4 to 5 minutes, until crispy and brown. Drain on paper towels and season with salt. Set aside and keep warm.

In a large bowl combine the 2 cups flour, cornstarch, baking powder, and 1 1/2 teaspoons salt. Make a well in the center and add egg yolks. Gradually whisk in club soda, working your way out from the center to form a smooth batter.

Season about 1 cup flour with salt and pepper. Dredge the fish fillets in seasoned flour, coating both sides, then dip in batter. Fry a few pieces at a time in 375 degrees F oil for 4 to 5 minutes, until crispy and golden brown. Drain on paper towels and season with salt.

When fish is done, place parsley leaves in hot oil. Fry leaves for 15 seconds, until just crispy and slightly translucent. Remove from oil with a slotted spoon; drain on paper towels. Serve fish with lemon halves, crispy fried parsley leaves, and Homemade Tartar Sauce.

In a small bowl combine 1/2 cup store-bought mayonnaise; 1/2 cup sour cream; 2 tablespoons capers, chopped; 1/4 cup chopped cornichons; 2 tablespoons chopped fresh Italian flat-leaf parsley; 1 tablespoon finely chopped fresh tarragon; and 1/2 lemon, juice only. Set aside in the refrigerator for 10 to 15 minutes before serving to let the flavors come together.