Combine the rhubarb, raspberries, sugar, and flour in a saucepan and cook over medium heat until the juices thicken, about 5 to 7 minutes. Reduce the heat and simmer for 20 to 30 minutes, until the rhubarb is soft. Transfer to a 4-cup shallow ovenproof baking dish and set aside.
Preheat the oven to 475 degrees. Make the meringue.
Fill a clean pastry bag fitted with the giant open star tip with the meringue. Pipe out the meringue onto the raspberry mixture into a simple latticework topping. Bake until very lightly browned, about 5 to 7 minutes. Serve immediately.