Remove the fish from the refrigerator and allow it to reach room temperature before cooking. In three separate shallow bowls, place the flour, eggs, and breadcrumbs, seasoning each with ¼ teaspoon of kosher salt and a few grinds of pepper. Add the thyme leaves and fennel seeds to the breadcrumbs and mix well. Season the fish with the remaining ½ teaspoon of kosher salt.
Working with 4-6 pieces at a time, dredge in flour (shake of excess) dip in egg (shake off excess) then dredge in the herby crumbs. Pat to stick.
Warm a 12-inch frying pan with two tablespoons of olive oil over medium heat, get ready to fry! When the oil shimmers, place the first batch in the oil. Cook until golden on the first side, about 2-3 minutes, then flip and add a knob of butter. Cook until golden on the second side, then flip to brown any remaining sides, add a little extra oil to the pan if it begins to look dry. Transfer to a large paper towel-lined plate.
Wipe out the pan to get rid of any wayward crumbs, then go again, adding another 2 tablespoons of the olive oil and the rest of the fish, butter, etc., until it's all fried. Sprinkle over flaky salt.
SERVE WITH wedges of lemon, mayonnaise (add chopped pickles, capers, soft herbs for tartar). REHEAT leftovers in the oven/toaster oven or air-fryer.
