Make the crust: Preheat the oven to 425F. Place the walnuts, sugar, and cinnamon in a food processor. Pulse several times until the nuts are ground fine. Pour in the butter and pulse until combined.
Spoon 2-3 tablespoons of the crust mixture into the bottom of each of ten 4-oz jars and press down lightly.
Make the custard: Beat together the sugar, flour, egg yolks, and cream in a bowl until the mixture is thick and creamy. Stir in rhubarb.
Spoon about 1/3 cup of custard and fruit into each jar over the walnut crust.
Place the jars 2 inches apart on a baking sheet. Bake the pies for 12 minutes, then reduce the heat to 350F. Bake for 20-22 minutes more, until the custard is set.
Remove pies from oven and allow to cool. Serve warm.