In a large skillet, heat 2 tablespoons of the butter over medium-low heat. Add the onion, red pepper flakes, 1 teaspoon of the rosemary, ¼ teaspoon salt, and ¼ teaspoon black pepper and cook, stirring occasionally, until the onions are golden brown and soft, 20 to 25 minutes. Stir in the brown sugar and balsamic vinegar until the sugar is melted. Continue cooking until very deep in color, 5 to 7 minutes more. Remove from the heat and let cool. When cool enough to handle, chop finely and add to a medium bowl. Stir in the cranberry sauce.

Preheat the oven to 350˚F. Using a serrated bread knife, slice the sourdough loaf into ¾-inch–thick parallel slices, cutting only about three-quarters of the way through so the bottom remains intact. Rotate the loaf 45 degrees and repeat the cuts, creating a diamond pattern across the surface.

Combine the garlic with the remaining 6 tablespoons butter, 1 teaspoon rosemary, and ¼ teaspoon black pepper. Brush the garlic butter all over the loaf, making sure to get between slices.

Tuck the brie in between the slices of bread and spoon the cranberry sauce mixture over it. Sprinkle the pepper jack all over the loaf, getting in between the slices and on top of the bread. Wrap the bread in foil and place on a baking sheet. Bake until the cheese is melted, about 30 minutes.

Unwrap the bread and place back on the baking sheet. Continue baking until the cheese is bubbly and the bread is lightly crisp, 12 to 15 minutes.