Parmesan and Herb Frittata Sandwiches

0 Preheat the oven to 350°F. Generously coat an 8 x 8 x 2-inch pan with cooking spray. Arrange the ciabatta buns, cut side up, on a separate large baking sheet. Set aside.
1 Whisk the eggs, parsley, dill, dark scallion greens, feta, 1/2 cup Parmesan, 1/2 cup yogurt, salt, and several grinds black pepper in a large bowl. Set aside.
2 Heat the oil in a large skillet over medium heat until shimmering. Add the white and light green parts of the scallions and cook for 2 to 3 minutes. Add the garlic and cook for 1 minute. Add the spinach, one handful at a time, and cook, stirring, until wilted, 3 to 4 minutes. Remove from the heat and season with salt to taste.
3 Pour the spinach mixture into the eggs and whisk to combine. Transfer to the prepared baking dish and sprinkle with the remaining 1/2 cup Parmesan. Bake until the frittata is puffed and just set in the center, 20 to 30 minutes (this will depend on the exact size and material of your baking dish). Let rest at room temperature for 5 minutes.
4 Place the ciabatta buns in the oven and bake for 5 minutes until toasted. (For really toasty buns, broil quickly instead). 
5 Meanwhile, whisk together the remaining 1 cup yogurt with the zest and juice of the lemon. Slice the frittata into six equal rectangles. Spread yogurt onto both sides of toasted ciabatta buns, then assemble the sandwiches with the frittata, Parmesan shavings, and a handful of arugula.