Preheat the oven to 375°F and place the frozen pie crust shell on a sheet pan or in a pie dish.
Heat a large skillet over medium-high heat. Add the Italian sausage and cook, breaking up the meat with the edge of a spoon, until cooked through and no longer pink, about 5 minutes. Reduce the heat to low, add and arugula, piquillo peppers, and red pepper flakes. Toss until the arugula just wilts, about 2 minutes.
Drain off excess fat and transfer the Italian sausage mixture to the center of the pie crust shell and spread into an even layer. Set aside.
In a medium bowl, whisk together the eggs, milk, salt, and pepper until well combined and frothy.
Sprinkle ¾ cup of the Manchego cheese over the sausage mixture in the pie crust. Then, pour the egg mixture on top. Finish with the remaining 1/4 cup Manchego cheese.
Carefully transfer to the preheated oven and bake until the center is completely set, 35 to 40 minutes.
Remove from the oven and let cool for 5 to 10 minutes.
Finish with freshly chopped chives. Slice, serve and enjoy!