Combine flour, cornstarch, and spices in a medium bowl. In another bowl, mix olive oil with 1 1/2 cups cold water. Slowly whisk into flour mixture until smooth. Cover and chill at least 1 hour.
Pour 2 in. vegetable oil into a large, wide pot. Insert a deep-fry thermometer and heat over high heat to 375°, then reduce heat to medium to maintain temperature.
Meanwhile, whisk whites with a mixer until soft peaks form. Using a rubber spatula, fold whites into batter.
Dip 1 stalk broccoli rabe at a time into batter, then carefully lower into hot oil. Cook, turning once, until golden, about 5 minutes. Transfer to a paper towel-lined baking sheet. Sprinkle with salt while warm. Dip and fry lemon slices the same way. Serve both warm or at room temperature with lemon wedges.