Preheat the oven to 350 degrees F.
In a large, oven-safe pan, melt the butter over medium heat. Add the leeks and cook until they’re just starting to turn golden, about 5-7 minutes, add the garlic and continue cooking until just fragrant, another 45-60 seconds. Add the wine to the pan, and cook until the liquid is just lightly reduced, about 3 minutes.
Add the cream to the pan, and cook, stirring, until warm, about 3 minutes. Add the salt and the dill to the pan, turn down the heat a bit to medium-low, and cook, stirring until the sauce begins to bubble, about 2 minutes. Add the fish fillets to the pan and use tongs to flip them once or twice, so that they are completely covered in the sauce.
Add the lemon juice to the pan, then place the slices on top of the fish. Bake the fish until it’s tender, about 30 minutes. Using an instant read thermometer, test the fish to make sure it has reached at least 145 degrees in the thickest part of the fillet. To test the fish manually, poke a fork into one fillet, and twist; the fish should flake relatively easily.