To make the concentrated strawberry purée, add the strawberries, 1 tablespoon granulated sugar, lemon juice, and a pinch of salt to a blender canister or food processor. Blend until completely smooth.

Add the purée to a small saucepan. Simmer on medium low heat until the purée is reduced by half, stirring occasionally. It'll take about 20-30 minutes to reduce. Cool to room temperature before making the cake.

Preheat oven to 350 degrees and spray 3 8-inch round cake pans with non-stick spray and line the bottoms with parchment paper.

In a large bowl, whisk together the gluten free flour, cornstarch, xanthan gum, salt, baking powder, baking soda, and granulated sugar. Set aside.

Add the egg whites and cream of tartar to a separate, clean medium bowl. Beat with an electric mixer on medium-low speed until the egg whites hold a stiff peak. Then set the the egg whites aside.

Pour the wet ingredients into the dry ingredients and mix until just incorporated. Add half of the whipped egg whites to the batter and gently fold in with a spoon. When it's mostly folded in, add the remaining egg whites. Gently fold in until no white streaks remain. Make sure you're folding and not stirring/mixing so you don't deflate the egg whites. It may take a minute or two to fully incorporate.

Transfer the batter to the prepared pans, making sure it's distributed evenly and spread the batter in the pans. Bake at 350 degrees for 20-24 minutes.

Cool the cakes in the pan for 25 minutes then remove them from the pans and cool completely on wire racks. Once the cakes are cooled, frost them with the strawberry cream cheese frosting.