Preheat the broiler. In a large bowl, whisk together 1 tbsp. lemon juice, the lemon zest, 1 tbsp. EVOO, salt and pepper. Toss with the fennel and fronds. Set aside.
In a large, ovenproof skillet, heat 1 tbsp. EVOO over medium-high heat. Stir in the broccoli rabe with 1/4 cup water; season with salt and pepper. Cover and cook until the stems are crisp-tender, about 3 minutes. Drain and transfer to a plate.
Add the remaining 3 tbsp. EVOO to the skillet and heat over medium-high heat. Add the potatoes; season with salt and pepper. Cook, turning occasionally, until tender, about 8 minutes. Add the scallions and broccoli rabe; cook for 2 minutes.
Stir half of the cheese into the eggs; season with salt and pepper. Pour evenly over the vegetables. Cook until set around the edges of the pan but still jiggly in the center, about 5 minutes. Sprinkle with the remaining cheese. Transfer to the oven and broil until set and golden, 2 to 3 minutes.
Serve the frittata with the fennel salad.