In a small bowl, whisk cornstarch with 2 tablespoons cold water until combined. Add cornstarch slurry to liquid in pot and bring to simmer over medium-high for about 5 minutes until thickened, stirring occasionally. Season sauce with salt and pepper to taste.
To serve, top ribs with thickened sauce and chopped parsley. Serve over mashed potatoes or mashed cauliflower.