If you celebrate Passover or eat gluten-free, you'll agree: Baking without all-purpose flour is no piece of cake. Our test kitchen chefs tweaked this recipe 10 times to get it right. At first the cake turned out too dry and dense (like many flourless cakes). Upping the potato starch gave the cake a softer texture, but without any leaveners the result was like a thick cookie. Then the chefs discovered the secret: They folded beaten egg whites into the batter, and the cake became light and airy.
Make the cake: Position a rack in the lower third of the oven and preheat to 325 degrees F. Brush the bottom and side of a 9-inch springform pan with olive oil and line the bottom with parchment. Brush the parchment with more olive oil and dust with almond flour, tapping out the excess.
Whisk the almond flour, potato starch and salt in a medium bowl. Combine 1/2 cup sugar, 1/3 cup honey, the lemon zest, 4 egg yolks, vanilla and almond extracts, and the olive oil in a large bowl; beat with a mixer on medium-high speed until smooth and creamy, about 3 minutes. Reduce the speed to low; beat in the almond flour mixture until just incorporated.
In a separate bowl, beat the 6 egg whites with a mixer on medium speed until foamy, about 1 minute. Gradually beat in 1/2 cup sugar until stiff glossy peaks form, about 3 more minutes. Gently fold about one-third of the beaten egg whites into the batter, then fold in the rest until just incorporated (it's OK if some white streaks remain). Pour the batter into the prepared pan. Bake until the cake is golden and springs back when lightly pressed, 50 to 55 minutes. Transfer to a rack to cool completely. (Don't worry if the cake falls slightly in the center.)
Meanwhile, make the topping: Place the lemon slices in a small saucepan, cover with water and bring to a simmer over high heat, about 3 minutes; drain and return to the pan. Cover with fresh water; bring to a simmer and drain. Repeat one more time with fresh water. Return to the saucepan and add the remaining 1/2 cup sugar, 1/3 cup honey, the lemon juice and 1 cup water. Bring to a simmer over high heat, then reduce the heat to medium and cook, stirring occasionally, until the lemon slices are tender and the liquid is syrupy, about 20 minutes. Set aside.
Run a thin knife around the edge of the cake, then remove the springform ring. Use a spatula to transfer the cake to a serving plate (remove the parchment). Remove the candied lemon slices from the syrup with a fork. Brush the cake all over with some of the lemon-honey syrup, then top with the candied lemon, almonds and pomegranate seeds. Serve with the remaining syrup on the side.