Kielbasa & Sauerkraut Casserole

Ingredients

• Olive or sunflower oil
• 1 large onion, sliced in half and then thinly again (semi-circles)
• 1 1/2 tsp caraway or fennel seeds (optional to lightly toast)
• 1 large sweet/tart apple chopped to 1/2" pieces
• 1 cup dry white wine
• 4-6 links kielbasa, casings removed and cut into 1/2-inch slices
• 2 1/2 cups drained sauerkraut (or 5 cups fresh green cabbage, sliced to ribbons)
• 3 tbsp apple cider vinegar
• 1 tbsp spicy mustard
• A pinch freshly ground black pepper
• 2 large potatoes (yukon gold or sweet potato), about 1 1/2 pounds, thinly sliced
• 1 tbsp butter, melted
• 1/4 tsp salt