Preparation

• Position a rack in the center of the oven and heat to 450°F.
• Heat the butter and oil in a small pan over medium-high heat. Add the shallot and a pinch of salt and cook, stirring, until the shallot is brown and crisp, 2 to 3 minutes. With a slotted spoon, transfer the shallot to a paper towel-lined plate to drain.
• In a small bowl, combine the shallot, mayonnaise, tarragon, mustard, and 1/4 tsp. black pepper.
• In a shallow bowl or pie plate, combine the chips, garlic, and cayenne.
• In a small bowl, beat the egg with 1 Tbs. water.
• Very lightly coat a rimmed baking sheet with some oil. Place the cod skinned side down on the baking sheet and pat dry with paper towels. Brush each piece with some of the egg wash, then firmly press on the potato chips. Bake until the fish is cooked through and the potato chips are browned, 8 to 12 minutes.
• Serve the fish with the sauce and lemon wedges.