To make tartar sauce: In the same skillet, add shallots and fresh garlic. Cook until brown and soft, and deglaze by adding Guinness and scraping up any bits and pieces at the bottom of the skillet. Stir often until Guinness is absorbed and skillet is mostly deglazed. Place shallots and fresh garlic in a mixing bowl and add mayo, vinegar, mustard, lemon juice, pickle and pickle juice, ½ teaspoon salt and ½ teaspoon pepper. Mix thoroughly and refrigerate until ready to serve.
To prepare potatoes: Add potatoes to a large pot and cover with water. Bring to boil, then completely drain water immediately. In separate bowl, combine ½ tablespoon salt, ½ tablespoon black pepper, granulated garlic, sugar and ½ tablespoon chili powder. Mix in with potatoes. In same cast-iron skillet, add remaining bacon grease, along with 2 cups peanut oil. Heat oil to 350 degrees before adding potatoes. Fry potatoes until golden brown, adding diced onion at end to brown. Remove potatoes/onions when golden; drain on paper towel.
To fry fish: In a bowl, combine flour, ½ tablespoon salt, ½ tablespoon pepper, granulated garlic, onion powder, 1 tablespoon chili powder and thyme. Dredge fillets through flour mixture. Heat oil in skillet to 350 degrees over medium heat. Fry fish on both sides until golden brown, then drain on paper towel.
If at home, reheat Brussels sprouts and potatoes for 10 minutes in 300-degree oven. Serve with fish, with tartar sauce on the side.