Spiced Cabbage with Blueberries (Kryddað Rauðkál með Bláberjum)

Ingredients

• 2 tbsp. unsalted butter
• 1 medium red onion, thinly sliced
• 2 cups fresh blueberries
• ¹⁄₂ cup red currant jam
• ¹⁄₄ cup red wine vinegar
• 2 tsp. ground cinnamon
• 1 tsp. ground ginger
• 1 large head red cabbage, cored and thinly sliced
• 1 tart green apple, such as Granny Smith, cored, peeled, and roughly chopped
• Kosher salt and freshly ground black pepper, to taste