Melt butter in large skillet over medium heat; cook onion for 3 minutes or until soft. Add rhubarb, sugar, lemon juice, mint and lemon zest. Bring to boil, stirring to dissolve sugar. Reduce heat and simmer, uncovered, for 5 to 7 minutes or until thickened. Season to taste with salt and pepper. Chutney can be prepared, cooled, covered and refrigetated for up to 2 days. Serve with lamb