Preheat oven to 375 degrees.
Cut stems away from any large kale leaves (the tender stems on younger shoots are fine to use) and then set aside. Bring a large pot of water to a boil, then blanch kale for about 4 minutes. Drain kale and allow to cool until you can easily handle it. Chop kale, squeezing out any excess water, and then place it in a large bowl.
Heat oil and butter in a large sauté pan and cook the onions over medium-low heat until tender (about 8 minutes). Add garlic and sauté for a few seconds. Then add onion and garlic mixture to the kale. Mix in cheese, eggs, salt, pepper, nutmeg, and walnuts.
Unfold one sheet of puff pastry and place on a baking sheet lined with a piece of parchment paper. (The paper should be slightly larger than the sheet.) Place kale mixture in the middle of the pastry, and then brush the pastry around it with cream.
Roll out the second pastry sheet on a floured surface until it is about 2 inches larger in all directions than the first sheet. Place this sheet over the top of the kale mixture and crimp edges. Brush top and crimped sides of the pastry with cream. Using a knife, make 3 or 4 slits in the pastry top and sprinkle with salt and pepper.
Bake for about 40 minutes or until pastry is golden brown. Transfer to a cutting board and serve.