extra-virgin olive oil and extra herbsfor serving, optional
Cook pasta according to directions in heavily salted water until one minute BEFORE it's al dente.
Meanwhile, melt two tablespoons of the butter in a large deep skillet or pot.
Add the three sliced green onions (white and green parts) and sauté over medium heat in the melted butter until softened (about 1 minute)
Add the frozen spinach to the skillet with the onions. Sauté until completely defrosted and most of the liquid has completely evaporated (about 3 minutes)
Meanwhile, in a small bowl, mix together the chopped fresh dill or other herbs (1/4 cup), the eggs (2), lemon zest, about half of the crumbled feta, and 1/4 teaspoon black pepper.
When the pasta has finished cooking to almost al dente, reserve 2 cups of the cooking water and add the pasta to the skillet with the spinach.
Continuing to cook over medium heat, add the feta, egg, and dill mixture to the skillet, as well as 1/2 cup of the pasta water. Stir to coat, and continue stirring, until the egg has cooked and covered every part of the pasta. If the sauce seems too thick or the pasta seems too dry, add it more pasta water 1/4 cup at a time (you'll probably need about 1 cup pasta water total). The pasta will finish cooking in this liquid and the sauce will thicken up- this will likely take about 3-5 minutes.
When the sauce has thickened and the pasta has completely finished cooking, turn off the heat and stir in the remaining two tablespoons of butter and the lemon juice until butter has melted. Top with remaining crumbled feta, extra fresh dill, olive oil, and season to taste.
To use fresh spinach, add it to the skillet according to the directions and cook until wilted and most of the liquid has evaporated. You can use whole baby spinach leaves or chopped mature spinach for this.
You can use other greens, such as kale, instead of spinach. Chop it up really finely and cook it for a bit longer so it's very tender. Arugula would work nicely as well.
I found this recipe didn't need any added salt, since the feta is salty and the cooking water from the pasta is as well.
If you don't have eggs, just leave them out. Add the pasta water to the skillet as directed, and add the feta, lemon zest, pepper, and dill directly to the pasta, stirring to melt all together.
You can use olive oil instead of butter, if you prefer.
Feta substitutions: goat cheese, queso fresco, and cotija cheese are similar to feta and can be used as a substitute.