Preheat your oven to 375°F (190°C). Heat an 8-inch cast-iron skillet over medium heat and add the olive oil.
Add the sliced potatoes with a pinch of salt. Cook in a single layer, stirring occasionally, until golden, crispy, and tender — about 6 minutes. Transfer to a plate.
Meanwhile, bring a small saucepan of water to a boil. Blanch the peas for 2 minutes, then transfer to an ice bath. Drain and set aside.
In the same skillet, add a little more olive oil if needed. Sauté the fennel for about 3 minutes until lightly caramelized. Add the scallions and cook for 1 more minute. Stir in the spinach, season with salt, cover, and let wilt for 30 seconds.
Return the cooked potatoes and peas to the skillet. Stir everything together evenly.
In a medium bowl, whisk together the eggs, milk, and about 1 teaspoon salt until well combined.
Pour the egg mixture into the skillet over the vegetables. Let sit over low heat for 1–2 minutes without stirring. Dot the goat cheese evenly on top.
Transfer the skillet to the preheated oven and bake for 10–12 minutes, or until the eggs are fully set and the center is firm.
Remove from the oven and let cool slightly. Top with minced chives, freshly cracked pepper, fresh arugula, and thinly sliced red onion. Slice and serve warm.