Position a broiler rack 3 inches from the heat and preheat the broiler. In a large, ovenproof skillet (preferably nonstick or well-seasoned cast iron), heat 2 tbsp. EVOO over medium heat. Add the potatoes and scallion whites and cook, stirring occasionally, until the potatoes are just tender, about 8 minutes. Transfer to a plate with a slotted spoon.
Add 2 tbsp. EVOO to the pan and heat over medium-high heat. Add the garlic and cook for 30 seconds. Add the broccoli rabe, season with salt and pepper and cook, tossing, until the leaves are wilted and the stems are crisp-tender, 2 to 3 minutes. Reduce the heat to medium. Return the potatoes to the pan, add the scallion greens and gently smooth the veggies into an even layer.
Whisk together the eggs, cheese, 1 tsp. salt and pepper to taste. Pour the egg mixture into the skillet and cook until set around the edges of the pan but still wet and jiggly on top, 6 to 8 minutes. Broil until just set, 2 to 3 minutes.
Meanwhile, whisk together the lemon juice, mustard and remaining 1 tbsp. EVOO. Just before serving, toss the spinach with the dressing; season with salt and pepper.
Run a knife around the edge of the pan to loosen the frittata. Cut into wedges and serve with the spinach salad.