Heat the olive oil in a heavey saucepan over low heat.
Add the onion and stew gently, stirring occasionally, until softened but not brown,about 7 minutes.
Stir in the rhubarb, tomatoes, honey, sugar, salt, and allspice.
Simmer uncovered, stirring occasionally, until it has melted down to a thick puree, about 20 minutes.
Add the lemon juice and salt and pepper to taste. The sauce should have a nice balance between sweet and sour, if not adjust seasonings.
Serve hot, over chicken or fish.