Oil a 9x13 inch baking pan and reserve. Preheat the oven to 350 F.
In a large bowl, combine the rolled barley, flour, brown sugar and salt. Drizzle in the coconut oil and toss to mix in, then stir in the milk. Scoop out 2 cups of the chunkiest part into a medium bowl, and toss with the almonds. Drizzle in a little more milk as needed to make the remaining clumps into a dough. Press into the pan.
In a medium pot, combine the rhubarb and sugar and place over medium heat. Bring to a simmer as the juices come out of the rhubarb. When the rhubarb is softened and juicy, whisk the milk and arrowroot in a cup, then stir into the simmering rhubarb. Stir until thick. Take off the heat.
Spread the rhubarb over the bottom crust, the top with the almond mixture.
Bake for about 40 minutes, until the juices are bubbling up around the edges and the topping is browned. Cool on a rack. For the neatest slices, refrigerate until completely cold. Cut 4 by 6.