Butter a 9-inch pie plate or tart pan. Preheat the oven to 425 degrees.
Cream the butter, 1 tablespoon sugar and orange zest in a mixing bowl or the bowl of a food processor. Add 1 cup of flour and mix until the dough just forms. Press evenly onto the bottom and the sides of a 9-inch tart pan or pie plate.
Prepare the rhubarb filling
Combine ½ cup sugar and ¼ cup flour with the rhubarb, toss, mixing well. Place on top of crust. Bake 10 minutes at 425 degrees.
Remove from oven. Lower oven temperature to 350 degrees.
Prepare the cream cheese filling
Beat filling ingredients in bowl with mixer.
Pour filling over the rhubarb and bake 30 – 35 minutes.
Prepare the topping
Whisk topping ingredients together in bowl.
Remove tart from oven and spread topping evenly over the tart. Chill for several hours before serving.