Preheat a 9″ cast iron pan over medium-high heat, and preheat the oven to 350ºF.
Whisk the eggs, sea salt, and pepper together in a medium sized bowl until combined, set it aside.
To make the decorative spiral of asparagus on top, cut the tops off 10 asparagus spears, measuring about 4 inches. Chop the rest of the asparagus into 1/4″ slices.
Add the butter or avocado oil to the preheated pan along with the 4″ asparagus spears. Sauté the spears just until crisp tender, remove them from the pan and set aside.
Add the rest of the thinly sliced asparagus and green onions to the pan. Sauté just until crisp tender, then add the egg mixture, basil, and about 1/3 of the fontina to the pan. Stir to thoroughly combine the sautéed ingredients into the egg mixture, then let the frittata cook untouched for 5-6 minutes. While the frittata is cooking, place the reserved asparagus spears in a wheel spoke pattern around the top, and sprinkle with the remaining fontina.
When the egg mixture is starting to set around the edges, transfer it to the oven and bake until the frittata is completely done, about 10 minutes
Serve immediately or at room temperature. You can also refrigerate this for up to 1 week, and reheat pieces as needed.