Potato, Apple and Onion Gratin - SippitySup

Ingredients

• 6 tablespoons unsalted butter, at room temperature, divided
• 1 pound red onions, peeled, halved & sliced
• 1 tablespoon chopped fresh rosemary leaves
• 3 teaspoons kosher salt, divided
• ⅓ cup water
• ⅓ cup dry white wine
• 1½ pound klondike rose, or similar idaho potatoes, sliced into ¼‑inch rounds
• 1 pound tart red apples (such as granny smith, pippin, or pink lady), peeled, halved, cored, cut into ¼‑inch-thick slices
• ¼ cup dried breadcrumbs
• ½ teaspoon freshly cracked black pepper