In a medium-size bowl, place the sliced strawberries and sprinkle the granulated sugar and (optional) vinegar on top. Toss gently to combine without crushing any strawberries.
Allow the mixture to sit while you make the cake so the strawberries release their juices.
Prepare the cake
Preheat your oven to 350°F. Grease an 8-inch or 9-inch round cake pan and line the bottom with a round of parchment paper (trace the perimeter of the pan on the parchment, then cut out the circle). Set the pan aside.
In a large measuring cup or medium-size bowl, place the egg, egg white, milk, and vanilla. Whisk to combine very well, and set aside.
Into the bowl of your stand mixer fitted with the paddle attachment (or a large mixing bowl with a handheld mixer), sift (yes, you have to sift!) the gluten free flour blend.
To the mixing bowl, add the xanthan gum (if needed), baking soda, baking powder, salt, and sugar, and whisk to combine well.
Add the butter and oil to the large mixing bowl, and mix on medium speed until the mixture appears uniformly sandy (about 2 minutes). Stop the mixer a few times to scrape down the sides to make sure all of the flour gets moistened by the butter and oil.
With the mixer on low speed, add the whisked wet ingredients.
Once the wet ingredients have been added, increase the mixer speed to medium and beat until the mixture seems mostly uniform (about 2 minutes). Using a silicone spatula, turn over the batter a few times by hand.
The batter should be fluffy and relatively smooth, although a few lumps are fine. Do not overmix the batter in an attempt to make it perfectly smooth. It will be thickly pourable.
Transfer the batter to the prepared pan, and smooth into an even layer with an offset spatula. Bang the bottom of the pan flat on the counter a few times to break any large air bubbles.
Place the baking pan in the center of the preheated oven and bake for 20 minutes. Rotate the pan in the oven 180°, and reduce the oven temperature to 325°F.
Continue to bake at 325°F until the cake is lightly golden brown all over, has begun to pull away from the sides of the pan and a toothpick inserted in the center comes out with no more than a few moist crumbs attached (at least another 15 minutes for an 8-inch cake, less for a 9-inch cake).
Remove the cake from the oven and place the pan on a wire rack. Allow the cake to cool in the pan for 15 minutes on the rack before turning it over onto the rack (and removing the parchment paper liner) and letting it cool completely.
Prepare the whipped cream
Place the heavy whipping cream and confectioners’ sugar in a large mixing bowl or the clean bowl of a stand mixer fitted with the whisk attachment.
Whip on medium-low speed in the stand mixer or using a handheld mixer until the cream just begins to thicken. Increase the mixer speed to medium or medium-high and continue to whip until the mixture holds a stiff (but not dry) peak.
Place the whipped cream in the refrigerator to chill while you finish the cake.
Assemble the cake
Once the cake has cooled completely, place it (bottom side up) on a cake plate or other serving plate.
Drain as much of the juice that the strawberries have released into a jar or small container.
Pour evenly over the top of the cooled cake. Pour the juice slowly so the cake has a chance to absorb the moisture. You can also use a spatula to help move the juice around and help the cake absorb it.
Remove the whipped cream from the refrigerator and spread it into in a single swoopy layer on top of the cake.
Top with the sliced strawberries and remaining juice. Serve immediately.