Preheat oven to 350 degrees F. Spray a 9-inch springform pan with non-stick spray.
In the bowl of a stand mixer fitted with a paddle attachment, beat butter and brown sugar until light and fluffy (about 3-4 minutes). Scrape down the sides of the bowl. Add egg and beat well until combined. Beat in lemon zest and vanilla bean paste.
In a medium bowl, whisk together the flour, baking powder, and salt. With the mixer on low speed, slowly add the flour mixture alternating with buttermilk, beginning and ending with the flour mixture, until just combined. Careful not to over mix. (I stop when there are still some streaks of flour because the next step involves more mixing.)
Quickly toss together the rhubarb and strawberries with 1 tablespoon of flour (this prevents sinking). Add to the batter and fold gently!
Pour batter into prepared pan. It is a think batter. Use the back of a spoon to spread out the batter evenly. Top with sliced almonds.
Bake for 45 minutes or until a toothpick comes out clean. Allow the cake to cool. Dust the top with powdered sugar right before serving. Enjoy!