"Fish And Chips" Casserole - cooking recipe

Ingredients

1 1/2 tablespoons unsalted butter
1 1/2 tablespoons all-purpose flour
3/4 cup whole milk
3/4 cup creme fraiche
1 1/2 teaspoons minced garlic (though I use 3 large cloves)
kosher salt
fresh ground black pepper
2 fresh thyme sprigs, push
1 tablespoon chopped fresh thyme (to garnish)
1 lb of 3/4-inch thick cod fish fillets or 1 lb haddock fillet, work well too
1 lb yukon gold potato

Preparation

Arrange a rack in the middle of the oven and preheat to 400 degrees. Generously butter a 2 quart shallow baking dish.
Melt the butter until hot in a medium saucepan over medium heat. Add the flour and cook, whisking constantly for about 2 minutes. Gradually pour in the milk and the creme fraiche, whisking constantly until the mixture comes to a simmer.
Remove from the heat and stir in the garlic, 3/8 tsp of salt and 3/8 tsp of black pepper. Add the thyme and let the sauce infuse with the seasonings for at least 10 minutes. I didn't have fresh available so I used about 1/4 tsp of dried thyme in the sauce and didn't use any additional for garnish.
Cut the cod or haddock fillets into 1 inch chunks and pat them dry with paper towels. Place the fish in the prepared baking dish and season with salt and pepper. Remove the thyme sprigs from the sauce (or leave in the dried thyme if you used that instead) and pour half of the sauce over the fish. Stir gently so that all the fish pieces are well coated and spread them evenly in a single layer.
Peel the potatoes and cut them crosswise into very thin slices (1/8 inch or less - I used a mandolin to do this). I ended up using less than 1lb once they were thinly sliced as about 2 medium sized potatoes ended up being enough to cover the 2qt. casserole dish even when overlapping a good amount.
Arrange the potato slices in a single overlapping layer on top of the fish. Season the potatoes with salt and pepper.
Pour the remaining sauce over the potatoes and spread evenly. (Tip: The casserole can be prepared up to this point about 4 hours ahead; cover and refrigerate. Bring to room temperature 30 minutes before baking).
Bake the casserole, uncovered, for 45 to 50 minutes. After 30 minutes the potatoes should have just started to brown and the liquids in the dish will be bubbling up slightly. At this point, start checking the casserole often until the potatoes are tender when pierced with a sharp knife and are golden.
Arrange a rack in the oven 4 to 5 inches from the broiler and broil the casserole until the potatoes are more browned - 1 to 2 minutes. Watch carefully as the potatoes can burn quickly.
Remove casserole from oven and let stand for 5 to 10 minutes and then sprinkle with thyme before serving.