From the "Christmas with Southern Living 2001" cookbook. It is fabulous! Great for holiday dinners. The flavors of the fennel and leek are wonderful.Goes well with any meat. Everyone loves it.
Rinse and trim off stalks of fennel. Discard the hard outer covering. Slice off bottom of bulbs and remove the center core. Slice the bulb lengthwise into 1/4" slices.
Remove outer covering from the leeks and trim off the dark green sections. Slice remaining into 1/4" slices.
Sauté fennel and leek slices in butter over med-high heat about 5 minutes in a large skillet. Add 3/4 tsp salt and 1 tsp thyme cover and simmer for about 10 minutes. Set aside.
Combine potato slices, flour and 1/2 tsp salt in zip-top bag and shake to coat. Arrange half of these slices in a well buttered 13"x9" baking dish.
Mix whipping cream and milk. Pour half of this mixture over potatoes in pan. Sprinkle with 1 1/2 cups of Gruyere cheese and 1 tsp of thyme. Spoon sautéed fennel/leeks over cheese and top with remaining potatoes. Sprinkle with remaining cheese, 1 tsp thyme, and pepper.
Bake, covered, at 400 degrees for 45 minutes. Uncover and bake an additional 15 minutes or until browned. Let stand 10 minutes.