In the body of a food processor, combine the almonds, sugar, rice flour and salt. Process until the almonds are finely ground. Add the softened butter, eggs, vanilla and almond extracts and process to a paste. If the paste is very runny, cover and chill for 20-30 minutes until firm.
Shape the tart:
If the dough is cold, let it stand at room temperature until slightly softened, 5 minutes in a warm kitchen or 15 minutes in a cool kitchen. Sandwich the dough between two pieces of parchment paper dusted lightly with oat flour, and gently begin pressing it flat, then rolling it into a 12x16" rectangle. As you work, periodically peel back the top piece of parchment, dust the dough lightly with flour, replace the parchment, grasp the dough sandwich with both hands and flip the whole thing over. Peel off the new top piece of parchment, dust with flour, and continue to roll. If the dough is uneven, cut off the long bits and press them onto the short bits, rolling to adhere. When your rectangle is 12x16", trim the sides so that they're even and straight. (If your dough becomes soft or sticky at any point, slip it onto a baking sheet, parchment and all, and chill it for 10-20 minutes to firm the butter.)
Slide the dough onto a sheet pan, still on the parchment. Spread the almond paste over the dough, leaving a 1" border all the way around. Gently fold over the edges to make a crust. Chill the dough until firm, 30 minutes.
Position a rack in the lower third of the oven and preheat to 400ºF.
Trim your rhubarb stalks the length of the dough, then cut them in half lengthwise (or in quarters if they're very fat). When the dough has chilled, lay the rhubarb across the dough, placing the stalks fairly close together. Brush the edges of the tart with the beaten egg. Sprinkle the sugar all over the rhubarb and crust.
Bake the tart until the almond paste is golden and puffed and the rhubarb is tender, 30-35 minutes. Remove from the oven and let cool. Just before serving, drizzle all over with the honey, and cut into 12 rectangles.
The tart is best the day of baking. It will keep at cool room temperature for up to 1 day, and refrigerated for up to 3 days.
Adapted from my Rustic Citrus Almond Tart, which I adapted from The Big Sur Bakery Cookbook.If gluten isn't an issue for you or your guests, make this tart with my wheat-based Flakiest, All-Butter Pie Dough instead, and use all-purpose wheat flour in the frangipane instead of the rice flour.In the filling, I used two smallish eggs, plus a third egg for brushing the edges of the tart. Alternatively, whisk two large eggs together in a bowl. Measure out 2 tablespoons and set them aside for brushing the dough, then use the remaining beaten egg in the filling.Nutritional values are based on one of twelve servings.