Using a sharp knife and cutting board, mince the sweet onion.
Make the Salad
Place the frozen mixed vegetables in a medium glass bowl and microwave on high for 3-5 minutes until crisp-tender.
Stir in the kidney beans and minced onion.
Prepare the Sweet and Sour Sauce
In a small saucepan over medium heat, bring granulated sugar, cider vinegar, and cornstarch to a boil, whisking constantly until thickened.
Let cool for 5 minutes and pour over vegetables. Season with kosher salt and freshly ground black pepper. Toss to combine. Refrigerate for 2-4 hours. Serves 4-6.