For the Riesling cake:
Butter or nonstick cooking spray, for pans
3 1/4 cups (425 grams) cake flour, plus more for the pans
1 tablespoon plus 1/2 teaspoon baking powder
3/4 teaspoon fine salt
1 cup (2 sticks/225 grams) unsalted butter, at room temperature
2 cups (400 grams) granulated sugar
6 large egg whites
1 1/2 cups (360 milliliters) sweet Riesling
For the rhubarb strawberry compote:
1 3/4 cups (8 ounces/225 grams) fresh strawberries, hulled and quartered
1 cup (4 ounces/115 grams) fresh rhubarb, cut into 1/4-inch (6-millimeter) pieces
1/2 cup granulated sugar
2 tablespoons fresh lemon juice
For the buttercream:
1 cup (240 milliliter) large egg whites
2 cups (400 grams) granulated sugar
3 cups (6 sticks/675 grams) unsalted butter, at room temperature
1 tablespoon pure vanilla extract
For the oat crumble:
1/2 cup (45 grams) quick-cooking oats
1/4 cup (25 grams) sliced almonds
1/4 cup (55 grams) firmly packed brown sugar
1/4 cup (30 grams) all-purpose flour
3 tablespoons unsalted butter, at room temperature
2 tablespoons honey
1/2 teaspoon ground cinnamon
1/4 teaspoon fine salt
For assembly:
Remaining buttercream
Fresh strawberries for decorating (optional)
Make the Riesling cake: Arrange 2 racks to divide the oven into thirds and heat to 350°F (175°C). Grease and flour 3 (8-inch or 20-centimeter) round cake pans and set aside.
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.
Make the rhubarb strawberry compote: Com