Preheat oven to 350°F.
Bring a large pot of salted water to a boil.
Remove the entire core of the cabbage with a sharp knife. Fill the sink or a very large bowl with cold water and ice. Place cabbage in boiling water and peel away the outer 8 to 10 leaves with tongs as they loosen, 2-3 minutes. Cold shock then gently remove leaves to kitchen towels and pat dry. Continue to boil the core for 3-5 minutes more then remove and let drain until cool enough to handle. Halve the core of the cabbage and thinly slice or shred. Wring out cabbage core pieces with a clean kitchen towel.
Place cabbage core in a mixing bowl and dress with about 3 tablespoons of EVOO, 3 to 4 grated garlic cloves, the fennel pollen or seed, sage leaves, a few grates fresh nutmeg and onion. Season with salt and pepper, and toss to combine.
Rub the bottom of a 10-inch round casserole or baking dish with butter. Line the dish with the largest and prettiest cabbage leaves, letting the edges hang over the dish a bit.
Add half the cabbage and onion mixture to the lined casserole dish and top with half of the sausage, making a solid, even layer and packing the dish down firmly. Add a couple more leaves of cabbage to create another layer, half the core filling and remaining sausage. Press layer down then top with a few more leaves of cabbage to seal filling. Fold in sides of first layer to secure cake and dot the top with butter.
Bake the cake in center of the oven for about an hour and 15 minutes, or until the center reaches an about 165°F. Cool cake about 15 minutes.
Place a platter over the casserole and carefully invert to unmold. Slice into wedges and serve with garlicky mashed potatoes alongside and lots of mustard for dipping.
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