Preheat the Oven: Adjust a rack near the top of your oven and preheat it to 300°F.
Cook the Asparagus: Heat a heavy, well-seasoned cast-iron or oven-safe skillet (8- or 9-inch is ideal) over medium heat and melt the butter. Add the asparagus along with ¼ teaspoon of kosher salt, and cook for about 5 minutes, stirring occasionally, until the asparagus becomes tender and develops some golden-brown spots.
Prepare the Egg Mixture: While the asparagus is cooking, crack the eggs into a large mixing bowl and whisk until well combined. Stir in the remaining ¾ teaspoon of kosher salt and the ¼ teaspoon of black pepper.
Combine in the Skillet: Pour the egg mixture into the skillet with the asparagus, stirring gently to distribute the vegetables evenly. Add dollops of ricotta over the top and scatter the minced chives. Let the skillet sit on the stove for about 2 minutes, just until the bottom layer begins to set.
Bake the Frittata: Carefully transfer the skillet to the preheated oven. Bake until the eggs are cooked around the edges but remain slightly creamy and custard-like in the center. This typically takes 12–15 minutes for a 9-inch skillet and 15–20 minutes for an 8-inch skillet. Check regularly toward the end of the cooking time to avoid overbaking.
Serve the Frittata: Once cooked, gently slide the frittata out of the skillet onto a serving plate. Drizzle with a little olive oil and a squeeze of fresh lemon juice for added flavor. Sprinkle with grated Parmigiano-Reggiano, flaky sea salt, and more freshly ground black pepper, if desired. Serve immediately or at room temperature.