- In a pot, place the chicken, leek greens, celery, bay leaf, Parmesan rind (if using) and water. Season with salt and pepper. Simmer over medium-low heat for about 40 minutes or until the chicken falls easily from the bones. Add water as needed so that the meat is always covered. Remove the chicken. Remove and discard the bones and skin. Shred the meat. Strain the broth. Set aside.
- Meanwhile, in a pot of salted boiling water, cook the cauliflower. After 2 minutes of cooking, add the lasagna noodles to the pot and cook for about 8 minutes or until the pasta is al dente and the cauliflower is tender. Drain. Lightly oil the lasagna noodles and place on a work surface.
- Place the cauliflower in a food processor. Add 1 cup (250 ml) of the chicken broth and purée to a smooth and very thick sauce (add broth, if needed). Season with salt and pepper. Keep the remaining broth for another use.
- In the same pot that was used for cooking the broth, soften the leek and garlic in the butter. Add the spinach and cook until just wilted. Add the chicken meat and half the cauliflower sauce. Adjust the seasoning. Remove from the heat.
- With the rack in the middle position, preheat the oven to 350°F (180°C).
- Divide the chicken filling among the 4 lasagna noodles. Roll each noodle up around the filling. Place the rolls, seam side down, in two individual baking dishes. Top with the remaining cauliflower sauce and sprinkle with the cheese. Freeze at this stage, if desired.
- Bake for 20 minutes. Finish cooking under the broiler for about 3 minutes or until the cheese is golden brown. Let rest for 10 minutes before serving.