Boil a pot of water and add 8 large Savoy cabbage leaves. Reduce heat to medium. Cook, covered, until leaves are tender but not mushy, 7 to 10 min. Drain leaves and place on a kitchen towel to cool completely. Pour 2 cups vegetable broth into same pot. Bring to a boil, then add 1 cup bulgur. Reduce heat to medium-low. Simmer, covered, until bulgur is tender and liquid has been absorbed, 6 to 8 min. Melt 3 tbsp butter in a large frying pan over medium. Add 2 minced garlic cloves, 1 finely chopped onion and 2 227-g pkg sliced white mushrooms. Cook, stirring frequently, until onion is soft, about 5 min. Season with fresh pepper. Stir in cooked bulgur and 1 tsp chopped fresh oregano. Preheat broiler. Stir 2 cups passata with 1/2 tsp granulated sugar and 1/4 tsp salt in a medium bowl. Spread 1 cup of passata sauce on the bottom of a 9 × 13-in. baking dish. Fill each cabbage leaf with 1/2 cup bulgur filling. Roll into cylinders, folding up sides. Arrange, seam-sides down, in dish. Pour remaining sauce evenly over cabbage rolls. Broil in top third of oven until sauce is warmed through, 5 to 10 min.