This tangy condiment is delicious on top of sharp cheese on crusty bread. It's also delicious when paired with roast chicken and pork, which benefit from its tartness.
In a saucepan, heat oil over medium. Add onion and cook until softened, 5 to 6 minutes.
Add rhubarb, raisins, sugar, ginger, and cinnamon stick. Cook over medium, stirring occasionally, until rhubarb begins to break down, 6 to 8 minutes. Remove cinnamon stick and stir in vinegar. Serve with ham or pork and cornichons.