In a blender or food processor, pulse together the flour, salt, and sugar a couple times to combine them.

Add the coconut oil and almond butter and pulse together until the mixture looks crumbly and holds together when you grab a handful of it and squeeze it.

Add the water and pulse until combined, adding a tablespoon or two more ice water if needed (if the dough doesn't hold together when you squeeze it in your hand, that's when you need a tad more water)

Transfer the dough onto a clean smooth surface and roll it out into a 1/4-inch thick circle. Transfer the circle onto a 9-inch tart pan and then press it into the pan to fit it snug. Trim off any excess dough that hangs over the outside edge of the tart pan.

Use a fork to prick the tart all over, then cover it with plastic wrap and refrigerate for at least 1 hour and up to 24 hours.

Strain the cashews and almonds and place them in a blender along with the sugar, almond butter, salt, and vanilla extract. Remove the center cap of the blender lid so that there is a small hole. Turn the blender on medium low speed and add the oat milk in one steady stream as it blends, pausing and stopping the blender as needed to scrape down the inside walls. Blend until everything is completely smooth.

In a small bowl, stir together the hot water and flax seeds and allow to rest for 10 minutes. The water should become slightly gelatinous at that point, and go ahead and stir the flax water mixture into the cashew mixture until combined. Set aside.

Set a box grater on its side and very carefully push the rhubarb stalk through the long slit on the top of the grater to cut a very long thin slice of rhubarb, about 1/8-inch thick. Repeat with all the stalks. (See the video for this process in action).

Bring a large pot of water to a slow boil. Add the rhubarb ribbons and stir. Cook for only 2 minutes, then strain them and immediately rinse them with cold water until the ribbons are cool to the touch. (If you don't do this the ribbons will continue cooking and become mushy and very difficult to braid.)

Take three of the ribbons and braid them on a clean flat surface. Repeat until you have braided all the rhubarb ribbons.

For the honey syrup, stir together the honey and water in a mug with a fork until the honey has dissolved. Set aside.

Preheat the oven to 375 degrees Fahrenheit.

Fill the tart crust with the custard mixture, leaving about 1/2-inch of space between the top of the custard and the top of the crust. Transfer each of the braided rhubarb ribbons to the top of the custard, arranging in your desired pattern (spiral, etc).

Place the pan in the oven and bake until the crust turns lightly golden, about 45 minutes. Remove and allow to cool for 20 minutes before slicing and serving.

While cooling, brush the braided rhubarb on top lightly with the honey syrup.