1. Peel the potatoes and plunge them into a pot of salted boiling water.
2. Preheat the oven to 200°C (392°F).
3. In a bowl, mash the potatoes.
4. Add the sour cream, salt, and pepper. Mix well.
5. Add the chopped chives, minced shallot, and diced ham.
6. On a baking sheet lined with parchment paper, unroll the first puff pastry sheet.
7. Spoon the mashed potato mixture onto the pastry in a ring shape.
8. Cover with the second puff pastry sheet, making sure to seal the edges. Lightly press the edges with a fork.
9. Place the Mont d'Or cheese in the center of the ring.
10. Using a knife, make regular cuts all around the ring, around the Mont d'Or.
11. Rotate each piece of pastry a quarter turn, so that the mashed potato is facing upwards.
12. Brush the tart with egg yolk and bake for 40 minutes.