Fresh Fig, Walnut, and Rosemary Upside-Down Cake

INGREDIENTS

• 4 tablespoons fresh lemon juice
• 1/2 cup unsalted butter
• 3 eggs, separated
• 1 teaspoon baking powder
• zest of 2 lemons
• 1 cup granulated sugar
• 10 fresh, ripe figs of your choice, tips removed, halved
• 2 teaspoons finely chopped fresh rosemary, divided
• 1 cup flour, sifted*
• 1/8 teaspoon salt
• 2-3 tablespoons walnut pieces, or as much as needed
• 1 cup light brown sugar
• 1 teaspoon vanilla extract