Place the oven rack in center of oven and preheat to 400 degrees F.
In a medium bowl, whisk creme fraiche and whipping cream until combined.
In a large baking dish, approximately 9 by 13-inches, arrange 1/3 of potato slices, overlapping slightly. Generously season with salt, pepper, and some fresh thyme leaves. Drop 1/3 of the cream mixture in dollops over potatoes, and spread evenly, (layer will be very thin). Repeat with another layer. Layer for a 3rd and last time, covering the top of the potatoes with the cream mixture, then mini cauliflower florets, and generously season with salt, pepper, and remaining thyme.
Bake 30 minutes and reduce oven temperature to 350 degrees F. Continue to bake until top is golden brown, about 20 minutes longer.
Do your best to let gratin rest 15 to 20 minutes before serving. Garnish with thyme sprigs. Eat and enjoy!