Step 1Remove the outer leaves of the cabbage, choosing 12 of the largest. Blanch the leaves in plenty of rapidly boiling salted water just until they soften, about 2 minutes. Immediately remove them from the water and shock them in a bowl full of ice water to stop the cooking. Press them dry between sheets of paper towel. Cut away the thick white spine from the centers of the leaves and set aside.
Step 2Chop enough of the inner leaves to make 2 cups.
Step 3Heat the hazelnut oil in a large skillet over medium heat. When the oil is hot, add the mushrooms and curry powder and cook, stirring, until the mushrooms soften, about 5 minutes. Season with three-quarters of a teaspoon salt and one-fourth teaspoon pepper, or to taste, and set aside to cool briefly.
Step 4Heat the oven to 325 degrees and grease a 9- by 13-inch glass baking dish with olive oil. Spread one of the blanched cabbage leaves flat on a work surface with the outside of the leaf facing down. Season it lightly with salt and pepper. Spoon about 3 tablespoons of the cooked mushroom mixture into the middle of the leaf and wrap the leaf around it to make a little package. Place it in the glass dish seam side down.
Step 5Heat 1 tablespoon olive oil in a large skillet. Add the onion and cook until it softens, about 5 minutes. Add the chopped garlic, the reserved 2 cups of chopped cabbage and the chicken stock and bring to a simmer. Cover and cook 20 minutes.
Step 6Let the mixture cool slightly. Ladle the mixture into a food processor and puree it, then pour the mixture over the cabbage rolls. Bake uncovered 60 minutes, occasionally spooning the cooking juices over the top. Serve immediately.