In a large heavy saucepan, combine filling ingredients, stir well and bring to a simmer over medium heat until mixture begins to thicken, about 5 minutes. Reduce heat to low and cook, stirring occasionally until the mixture is jamlike and has reduced to about 1-3/4 cups, about 35 minutes. Transfer to a bowl, let cool, then cover and refrigerate until cold.
Position a rack in the center of the oven and preheat to 350°F. Generously butter two 8-inch round cake pans, line pan bottoms with parchment and butter the parchment.
In a large bowl, whisk together flour, baking powder and salt. In another large bowl, use a mixer with whisk attachment to cream butter with 1/2 cup sugar on high speed until light and fluffy, about 5 minutes, scraping down the bowl occasionally. Add egg yolks, one at a time while beating at medium speed to incorporate. Scrape down bowl and beat mixture for 2 minutes until bright yellow. In a small bowl, combine milk and vanilla.
Gradually add 1/2 of the flour mixture to the creamed butter/eggs and beat just until incorporated. Pour in milk mixture; mix at low speed until just blended and then add remaining flour, beating until incorporated. Divide the batter evenly between prepared pans, smoothing tops.
In a clean bowl, use a mixer with a clean whisk attachment to beat the egg whites at medium speed until frothy, about 2 minutes. Gradually sprinkle in the remaining 3/4 cup sugar, scraping down bowl as needed. Beat whites on high speed until glossy, medium peaks form, about 6 minutes. Divide the meringue evenly between the cake pans, spreading over the batter with the back of a spoon, leaving a 1/2-inch border all around uncovered. Do not smooth meringue tops; sprinkle sliced almonds over the meringue and bake until puffed and lightly browned, about 25 minutes. Let layers cool in their pans on a cooling rack; tops will sink considerably.
While the cakes are still warm, run a knife or small offset spatula around the sides of the cake layers to loosen them from the pans.
After the layers are cool, carefully invert the less attractive layer onto a baking sheet and remove the parchment. Invert onto a serving platter so meringue side is up. Gently spread the strawberry-rhubarb filling over the top and cover with the other cake layer, meringue side up.
The cake may be assembled to this point a day ahead and kept in the refrigerator, loosely covered with plastic wrap.
To serve, top with fresh strawberries, if desired and slice gently using a serrated knife.