Preheat the oven to 350 degrees F. Line a 9 x 13-inch pan with parchment paper, leaving a 1-inch overhang on each of the long sides. Spray the pan and parchment with baking spray.
Make the filling:
Combine the rhubarb, brown sugar, cornstarch, lemon zest, orange juice, and salt together in a bowl. Stir well and set aside.
Make the crust:
Put the almonds in the bowl of a food processor and run for about 20 seconds, or until the almonds are finely ground. Add the oats, flour, baking soda, salt, and sugar and pulse 12 times, or until combined. Add the butter and vanilla extract and run for about 2 minutes, or until the dough is thoroughly mixed and just starts to come together.
Put about half the dough, about 2 1/4 cups, in the prepared pan and gently press evenly over the entire bottom, using an offset spatula. Pour the rhubarb filling over the crust and dot with the 2 tablespoons butter.
Using your fingers, make some large clumps with the remaining dough and place them evenly over the rhubarb; sprinkle the rest of the dough crumbs over the filling so the entire surface is lightly covered with topping. Bake for 30 minutes, then increase the oven temperature to 375 degrees F and bake for another 8 to 10 minutes, or until the crust is pale golden brown.
Let cool and cut into 2 x 3-inch rectangles. Use an offset spatula to remove the bars from the pan.
Reprinted with permission from Sweet Home by Rebecca Miller, Kyle Books copyright (c) 2012