Heat the olive oil in a medium saucepan over medium-low heat. Add the red onion, mustard seeds and salt and cook, stirring often, until the onions are soft and translucent, about 10 minutes. Add the vinegar, sugar and Dijon mustard and stir to combine. Bring to a simmer and add the red grapes. Cook, stirring often, until the grapes are starting to break down but still have a little texture, about 10 minutes longer. Allow the mixture to cool completely. Serve with your favorite meats and cheeses.