Make the buckwheat crêpes according to the recipe, cover with foil and keep warm in a 200°F oven.

Heat a little olive oil in a large sauté pan. Season the chicken cubes with salt and freshly ground black pepper. Add the chicken to the pan and sauté until browned on all sides. Remove from the pan and set aside.

Add the butter to the pan along with the leeks and sauté for a few minutes until they start to soften. Add the mushrooms and a little more olive oil, if needed and cook until the mushrooms are lightly browned. Deglaze the pan with sherry, stirring to get all the browning off the bottom of the pan.

Return the chicken to the pan, lower the heat and simmer for 2 minutes. Stir in the heavy cream and lemon zest. Simmer for a few more minutes and then season with salt and freshly ground black pepper.

Assemble the crêpes. Sprinkle some of the fontina cheese in the middle of the warm buckwheat crêpes. Spoon some of the chicken filling mixture over the cheese. Roll the crepes up to cover the filling or spread the filling out and fold the crepes in half and then in quarters to form triangles.

Serve immediately.