In a large bowl, whisk together the eggs, Parmesan cheese, chopped pepperoncini peppers, parsley, and 1/2 tsp each of salt and pepper. Set aside.

Heat a 10-inch nonstick skillet over medium-high heat. Crumble in the Italian sausage and chopped onion. Cook for about 4 minutes, breaking up the meat, until cooked through and onion is translucent. Add garlic and cook 1 more minute until fragrant. Scrape into the egg bowl. Wipe skillet clean with a paper towel.

Add 3 cups water to the same skillet over high heat. Add the angel hair pasta broken in half, 3/4 tsp kosher salt, and 3 tbsp olive oil. Bring to a boil, stirring occasionally. Boil 8 to 12 minutes until all water evaporates and you hear the oil sizzling.

Reduce heat to medium. Stop stirring and use a spatula to lift pasta edges every minute or two to prevent sticking. Cook 5 to 7 minutes until the bottom is golden and crispy.

Push pasta up the sides of the skillet to cover the full surface. Pour the egg and sausage mixture over the top. Use tongs to gently lift sections of pasta so the egg flows toward the pan bottom without breaking the crispy layer.

Cover with a tight-fitting lid and cook on medium for 5 minutes. Edges should be set and bottom golden brown. If center looks very raw, cover and cook 2 to 3 more minutes.

Put on oven mitts. Place a large plate over the skillet and flip the frittata onto it, browned side up. Slide back into the skillet raw side down. Tuck edges back in with a spatula if needed.

Cook over medium heat until the second side is lightly golden, 2 to 4 minutes. Slide onto a plate, cut into wedges, and serve immediately.