Preheat oven to 400°F.
Skewer 3 potatoes on each rod, poking them toward the bottom side rather than through the middle. Cut very thin slices about 1/8-inch thick down to rod down the whole potato. Drizzle the potatoes with olive oil to lightly coat and season with salt and pepper. Roast 20 minutes to get the potatoes to open a bit along the slices.
Combine lemon zest, rosemary and garlic in a small bowl and sprinkle the mixture in between the potato slices. Roast 25 minutes more. Remove from oven and keep warm loosely covered with foil.
Melt butter in a small skillet and add panko. Toast until light and evenly golden; remove from heat. Cool to room temp and toss with parsley and half the cheese.
Working from the rounded thicker side of the chicken breast pieces, cut into the breasts to butterfly them, opening them like a book. Gently pound each piece to 1/4-inch thick. And season the inside of the meat with salt and pepper.
Combine defrosted spinach, separating it with fingertips, with ricotta, egg yolk, walnuts, salt, pepper, nutmeg and remaining cheese. Fill each breast with a few tablespoons of cheese each and close them. Drizzle with a little olive oil and season lightly with salt and pepper. Wrap each piece of stuffed chicken with speck.
Heat a large skillet with 2 tablespoons oil, 2 turns of the pan, over medium-high heat. Brown chicken 3-4 minutes on each side. When you remove the potatoes from the oven reduce heat to 350°F, add chicken and roast 15 minutes to finish cooking through.
Slice the stuffed chicken on a bias and fan piece out on warm dinner plates. Serve with lemon wedges with Hasselback potatoes alongside topped with parsley and cheese breadcrumbs.