Heat a large skillet over medium heat. Add oil; swirl. Add shallots; sauté 2 minutes. Add mustard seeds, ginger, garlic, and serrano; sauté 2 minutes. Stir in sugar and next 4 ingredients (through cranberries); bring to a simmer. Reduce heat and cook 4 minutes. Add mango; cook 1 minute. Stir in walnuts.
