This isn't technically a gratin, but I didn't know what else to call it. Regardless, it was a good one, and even better the next morning with fried eggs, brown butter, and extra capers.
Ingredients
1kg maris piper potatoes
2-3 fennel bulbs + fronds
1/4 lemon, juice
1 head radicchio
1 orange or 1-2 satsumas, juice
1 tbsp Dijon mustard
1 tbsp red wine vinegar
1 tbsp drained capers
4 tbsp EVOO for the gratin
2-3 tbsp EVOO for the dressing
Salt and pepper to taste
Trim the green stalks from the fennel and reserve the fronds. Thinly slice the potatoes and fennel with a mandolin. Add to a large bowl with olive oil, squeeze of fresh lemon juice, and salt, then mix well.
Benriner Mandoline 64 (this is an affiliate link)
Preheat the pan with a generous amount of oil. Use a heavy-bottomed, oven-safe pan (carbon steel or cast iron is best). I used a carbon steel, linked below.
De Buyer Mineral B Fring Pan (this is an affiliate link)
Layer in the potato and fennel mixture, forming at least a 6cm thick layer. Cook over medium heat for 8-10 minutes until a golden crust forms on the bottom. Tilt the pan slightly to check.
Add a cartouche (see instructions below), pressing it down firmly. Transfer the pan to a preheated oven at 165°C. Bake for 45 minutes to an hour to bind the layers and prevent burning.
Cartouche instructions
Cut a square of greaseproof paper larger than your pan.
Fold it into a triangle, then keep folding until you have a narrow point.
Measure from the centre of the pan to the edge, cut to size, then unfold.
Place it over the food, pressing it down lightly.
Cut the radicchio into quarters and remove the core.

Add the leaves to a mixing bowl with the fennel fronds. In a small bowl, whisk together orange or clementine juice, mustard, capers, and red wine vinegar. Add a pinch of salt and sugar to balance if needed.
Serve the gratin with the radicchio salad on the side. This whole situation if more then happy to have a generous amount of parm grated over it as well.