In a medium saucepan over high heat, heat butter and olive oil. Add onions and shallots; stir 2 to 3 minutes. Reduce heat to low and cook 18 minutes more, stirring occasionally. Add vinegar and cook 2 minutes more, until onions are golden brown; cool. Meanwhile, in a food processor or blender, purée cottage cheese until smooth. Add Blue cheese, cooled onions, tarragon, and milk; pulse to desired consistency. Season with salt and pepper. Stir in sun-dried tomatoes, if desired. Serve with fresh vegetables and crackers. Refrigerate leftover dip in an airtight container for up to 3 days. Makes 2-3/4 cups.