Combine rhubarb, apple, cranberries (or cherries), onion, water, honey, ginger, vinegar and crushed red pepper to taste in a small saucepan. Bring to a boil, stirring occasionally. Reduce heat to medium-low, cover and simmer until rhubarb is tender, 15 to 20 minutes. Uncover and simmer, stirring occasionally, until thickened, about 5 minutes more. Serve warm or cold.