Combine the plant-based milk and apple cider vinegar in a bowl and let it sit for 5 minutes—this curdling process creates a vegan buttermilk that will add tanginess and help the cake rise. While this sits, whisk together the oil, applesauce, sugar, vanilla extract, and orange zest in another bowl until well combined. After the milk has curdled, pour it into the wet ingredient mixture and stir to combine. This two-component approach prevents the vinegar from prematurely reacting with the dry ingredients.

Preheat your oven to 350°F. While it heats, whisk together the all-purpose flour, baking powder, salt, cinnamon, and ground nutmeg in a separate bowl. I like to do the dry mix now so I'm not waiting for it while the wet ingredients are ready—this keeps the batter from sitting too long and losing lift. Set the dry mixture aside until ready to combine.

While the oven continues heating, prepare a 9-inch round cake pan or similar size by melting the vegan butter and brushing it evenly across the bottom and sides. Sprinkle the brown sugar over the butter to create an even, caramelized base layer. The butter and sugar will create a rich, sticky coating that will become the glossy top of your cake when inverted. Make sure the sugar is distributed evenly to avoid any burnt spots.

Combine the hulled and sliced strawberries with the rhubarb pieces in a bowl, gently tossing to mix. Arrange them in an even layer over the butter and brown sugar base, pressing down lightly so the fruit sits in an attractive pattern—this becomes the visible top of the cake after flipping. The fruit will release juices as it bakes, creating a jammy layer between the caramelized sugar and the cake, so aim for fairly compact arrangement to prevent gaps.

Fold the dry ingredient mixture from Step 2 into the wet ingredient mixture from Step 1 using a spatula or wooden spoon, mixing just until the flour is incorporated—don't overmix, as this will develop gluten and make the cake tough. I find that a few lumps in the batter actually lead to a more tender crumb in upside-down cakes. Gently pour and spread the batter evenly over the prepared fruit layer, being careful not to displace the fruit arrangement.

Bake for 35-40 minutes, until a toothpick inserted into the cake portion comes out clean and the top is lightly golden brown. Remove from the oven and let cool for exactly 5 minutes—this gives the caramel time to set slightly but keeps it pliable enough to release from the pan cleanly. Run a thin knife around the edges of the pan to loosen any stuck bits, then invert the cake onto a serving plate in one confident motion, allowing any caramel to flow over the top.