In a large combine all the above ingredients together and let sit 1 hour covered.
Heat oil in a high sided sauté pan to about 350° F. Pat dry the cod fillets and coat evenly with the beer batter. Fry fillets in the hot oil until golden brown on both sides. Drain on paper towels. Serve hot with a generous amount of tartar sauce.
Using a food processor fitted with the sharp blade, add cornichons, capers, and shallots, to the work bowl. Process for a few seconds just to combine. Add the mayonnaise, mustard, herbs, lemon juice, and pepper, and process for 8 seconds to well. Open the pour spout, and with the motor running, slowly add the oil in stream.
Remove tartar sauce to an airtight container and refrigerate for 24 hours before using to allow flavors to develop.Makes 3 cups Source: