Heat 2 tablespoons oil in large nonstick skillet over medium high heat. Add onions and sugar. Cook until dark brown and tender, stirring frequently, about 16 minutes. Mix in vinegar and crushed pepper. Cook until mixture is thick, about 1 minute. Season marmalade generousl to taste with salt and pepper. Meanwhile, preheat oven to 425°F. Arrange bread on baking sheet. Brush with remaining 2 tablespoons oil; sprinkle with salt. Bake until crusty, about 8 minutes. Overlap 2 prosciutto slices on each toast. Top with 1/4 cup cheese, then marmalade. Garnish with rosemary.