Preheat oven to 400F and adjust rack to the middle of the oven.
Toss cheeses together in a medium bowl to combine. Transfer about 1/3 of the mixture to another bowl and set aside for later use. Back in the first bowl, add cream, salt, pepper, garlic, sage, & thyme; and stir or toss to combine. Set aside.
Using a mandoline slicer, slice all the potatoes, unpeeled, into 1/8 inch thick disks and place into a very large bowl. The larger the bowl, the easier it will be to toss the potatoes with the cream without making a mess. Slice the onion(s) and fennel bulb(s) to the same thickness and add to the potatoes. Pour cream and cheese mixture over the potatoes and toss toss toss to completely coat each slice of potato with cream. Use your fingers to separate any potatoes that may have become stuck together, so that every single slice is coated in the cream mixture.
Butter the inside of a large casserole, or 12? cast iron skillet *see note. Organize handfuls of potatoes into neat stacks, along with some slices of onions and fennel, and line them up in the casserole with their edges aligned vertically. Continue placing stacks of potatoes into the dish, working around the perimeter and into the center until all potatoes have been added. Potatoes should be tightly packed. If necessary, slice an additional potato, coat with the remaining cream mixture, and add to the casserole. Pour the remaining cream mixture evenly over the potatoes until the mixture comes about half way up the sides of the potato slices. You may not need all the liquid.
Cover dish tightly with a lid or aluminum foil and transfer to oven. Bake for 30 minutes. Remove lid/foil and bake for 30 minutes longer. Carefully remove from oven, sprinkle with remaining cheese, and return to oven to bake until deep golden brown and crisp on top, about 30 minutes longer. Remove from oven, let rest for at least 15 minutes, and serve.