Adjust oven rack to middle position and heat oven to 350 degrees.
Whisk eggs, milk, and ½ teaspoon salt in bowl until well combined. Stir in feta and oregano.
Heat 1 teaspoon oil in 12-inch ovensafe nonstick skillet over medium-high heat until shimmering. Add peas, pepper flakes, and ¼ cup water. Cover and cook until peas are bright green and just tender, 3 to 5 minutes. Uncover, stir in lemon zest and ½ teaspoon juice and cook until skillet is dry, about 1 minute.
Add egg mixture and cook, using rubber spatula to stir and scrape bottom of skillet until large curds form and spatula leaves trail through eggs but eggs are still very wet, about 30 seconds. Smooth curds into even layer and cook, without stirring, for 30 seconds. Transfer skillet to oven and bake until frittata is slightly puffy and surface bounces back when lightly pressed, 6 to 9 minutes.
Using rubber spatula, loosen frittata from skillet and transfer to cutting board. Let sit for 5 minutes. Meanwhile, whisk remaining 5 teaspoons oil, remaining 2 teaspoons lemon juice, and remaining pinch salt together in large bowl. Add arugula and fennel and toss to coat. Season with salt and pepper to taste. Serve.
