In a large saucepan over medium heat, cook strawberries and sugar, mashing berries occasionally with a potato masher until jam-like, about 10-15 minutes (mixture should equal about 2 ¼ cups).
While berries cook, in a small bowl mix together water, 2 teaspoons lemon juice, and gelatin, and let stand until gelatin softens about 5 minutes. Pour gelatin mixture into the hot strawberry mixture and stir until dissolved. Transfer to a large bowl, cover with plastic wrap, and refrigerate until set, at least 4 hours (up to overnight).
Move oven rack to lower-middle position and heat to 350 degrees F. Spray 12x18-inch rimmed baking sheet with nonstick cooking spray, line with parchment paper, and then lightly spray parchment paper with nonstick cooking spray.
In a medium bowl whisk together flour, baking powder, and salt; set aside.
In the bowl of a stand mixer fitted with the whisk attachment, whip eggs on low speed until foamy, about 2-3 minutes. Increase speed to medium and slowly add in sugar. Increase speed to high and beat until the eggs are pale yellow in color and very thick, about 5-10 minutes. Add vanilla and beat until combined.
Remove bowl from mixer, and sift flour mixture over egg mixture. Using a large rubber spatula, fold the flour in until no traces of flour remain.
Pour batter into the prepared cake pan and spread into an even layer using a large offset spatula. Bake until cake springs back when touched and feels firm, about 10-15 minutes.
Once cake comes out of the oven, run a paring knife around edge of cake to loosen, then flip the cake onto a large sheet of parchment (parchment should be longer than the cake). Carefully peel off the parchment paper from the cake, and roll cake and parchment up into a log. Move log to a wire rack and cool for 1 hour.
In a clean work bowl of a stand mixer fitted with the whisk attachment, whip cream, mascarpone, sugar, lemon juice, and vanilla on medium speed until stiff peaks form, about 3-4 minutes. Cover and chill until ready to assemble the cake.
Carefully unroll cake and use a small offset spatula to spread strawberry mixture over the top (strawberry mixture may need to be stirred before spreading over cake), leaving a 1-inch border around all edges. Clean off offset spatula, and repeat with cream (reserve 3 tablespoons of cream if you plan to garnish the cake with it).
Snugly reroll cake, leaving parchment behind. Wrap cake in plastic wrap and chill until firm, at least 4 hours.
Dust cake with powdered sugar, and top with reserved whipped cream and sliced strawberries (optional). Slice and serve.