Cut the butter and cream cheese into 1/4-inch cubes, place on a baking sheet and freeze until they are very firm but not frozen solid, about 20 minutes.
Using a standing mixer and the paddle attachment, mix the flour, Truvia, pine nuts, vanilla, lemon zest, cinnamon, ginger, nutmeg, and salt on medium-low just to combine.
Add the chilled butter and cream cheese on medium until they are in small, pebble-size pieces. (Alternatively, stir everything except the butter and cream cheese together in a bowl and cut in the butter and cream cheese with a pastry cutter or two knives.)
Freeze the topping mixture for 15 minutes.
About 20 minutes before baking, preheat the oven to 375oF. with a rack in the center position, and another in the lower third of the oven.
In a medium bowl, stir together the Truvia, flour, and cinnamon. Add the rhubarb and toss until it is evenly coated.
Divide the filling evenly among six 6-ounce ramekins or custard cups.
Place the filled ramekins (no topping) on a baking sheet and drape a single piece of foil over them. On a non-stick or lightly oiled baking sheet, spread the topping in an even layer. Place the sheet with the ramekins on the lower oven rack and the one with the topping on the center rack.
Bake the topping until it is golden, about 25 minutes, turning it with a spatula every 10 to 15 minutes. Bake the filling until it is bubbling, about 25 minutes. To serve, sprinkle one-third cup topping over each ramekin. Serve with sugar-free ice cream, if desired.