Place the potatoes on a sheet pan, drizzle olive oil on top, and season with salt. Bake the potatoes in a preheated oven set to 350°F for 45 minutes.
Cut 1/3 off the potato lengthwise, scrape the potato from the potato skin and add it to a bowl, and discard the potato skin. Repeat with the rest of the potatoes and mash the potatoes in the bowl with a fork.
Add the mashed potatoes to a casserole dish and press the layer down with a fork.
Partially hollow out the remaining 6 potatoes and add the scrapings to a bowl.
Add the ham, crème fraîche, and chives to the potato mixture in the bowl, season with salt and pepper, and mix well with a fork.
Fill the hollowed-out potatoes with the ham & potato mixture and press it down with a spoon.
Turn the potatoes over and place them on the potato bed in the casserole dish.
Cover everything with béchamel sauce and sprinkle shredded mozzarella cheese on top. Put the casserole dish in the oven and bake at 375°F for 25 minutes.
