1. Heat oven to 400˚.
2. Crumble cheeses together in bowl, set aside ½ cup of the mixture to top the casserole later.
3. Add cream, and thyme to cheeses, season liberally with salt and pepper (this liquid will season all the potatoes and fennel), then whisk to combine.
4. Add sliced potatoes and sliced fennel to the liquid and toss liberally until every slice is coated in cream and cheese.
5. Grease 2-quart casserole dish with butter. Grab stack of potatoes and fennel, organizing them into tight pile, then lay them in casserole dish standing on edge, instead of flat like pancakes.
6. Continue grabbing stacks of potatoes, pressing them together, and wedging them into the casserole dish until entire dish is full. The dish should be tightly packed.
7. Pour any excess cream/cheese mixture over the potatoes. You may not have any.
8. Cover dish tightly with foil and transfer to oven. Bake for 30 minutes.
9. Remove foil and continue baking until top is pale brown, about another 30-40 minutes.
10. Remove from oven, sprinkle with remaining cheese, and return to oven. Bake until golden brown and crunchy on top, about 30-40 minutes more.
11. Remove from oven to rest for a few minutes, garnish with fennel fronds, serve hot.