Sift together the flour, baking powder, and salt and set aside.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed until smooth. Add the sugar and mix on medium-high until the butter is light and fluffy, 3 to 5 minutes. Stop the mixer and scrape down the bowl.

Turn the mixer to medium-low and gradually add the vanilla and egg whites until combined. Stop the mixer and scrape down the bowl. Turn the mixer to low and add the flour mixture in three batches, alternating with the Riesling, beginning and ending with the flour mixture. Mix on medium for no more than 30 seconds after the last streaks of the dry ingredients are combined.

Evenly divide the batter among the prepared pans. Bake for 23 to 25 minutes, or until a toothpick inserted into the center of the cakes comes out clean, switching the cakes between racks halfway through. Let them cool on a wire rack for 10 to 15 minutes before removing the cakes from their pans.

Combine the strawberries, rhubarb, sugar, and lemon juice in a medium saucepan and cook them over medium-high heat, stirring intermittently with a wooden spoon, until the juices start to bubble.

Reduce the heat to maintain a simmer and cook for 8 to 10 minutes, or until the fruit starts to break down. Remove the pan from the heat and let cool.

Place the egg whites and sugar in the bowl of a stand mixer. Whisk them together by hand to combine.

Fill a medium saucepan with a few inches of water and place it over medium-high heat. Place the mixer bowl on top of the saucepan to create a double boiler. The bottom of the bowl should not touch the water. Whisking intermittently, heat the egg mixture until it registers 145°F to 155°F (63°C to 68°C) on a candy thermometer or is hot to the touch.

Once hot, carefully fit the mixer bowl onto the stand mixer. With the whisk attachment, beat the egg white mixture on high speed for 8 to 10 minutes, until it holds medium-stiff peaks. When done, the outside of the mixer bowl should return to room temperature and no residual heat should be escaping the meringue out of the top of the bowl. Stop the mixer and swap out the whisk attachment for the paddle.

With the mixer on low speed, add the butter, a few tablespoons at a time, and then the vanilla. Once incorporated, turn up the mixer speed to medium-high and beat until the buttercream is silky-smooth, 3 to 5 minutes.

Arrange a rack in the middle of the oven and heat to 375°F (190°C). Line a baking sheet with parchment paper.

In a medium bowl, mix together the oats, almonds, brown sugar, flour, butter, honey, cinnamon, and salt with a wooden spoon until combined. The mixture should resemble clumps of sand.

Sprinkle it over the baking sheet and bake, stirring halfway through, for 8 to 10 minutes, or until golden-brown. Let it cool and crumble the mixture into smaller pieces, if necessary.

Once the cakes have completely cooled, level them and choose which layer will be at the bottom. Place it on a cake plate or serving dish.

Spread on 1 cup of the rhubarb buttercream with an offset spatula. Sprinkle it with 1/2 to 3/4 cup (50 to 75 gramof the oat crumble. Top with the next layer of cake and repeat with the buttercream and remaining crumble, finishing with the final layer of cake.