1. 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.
2. 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.
3. Meanwhile, whisk whites with a mixer until soft peaks form. Using a rubber spatula, fold whites into batter.
4. 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.
Nutritional analysis is per serving.