Bring a large pot of salted water to a boil.
Cook pasta according to package instructions.
Reserve 1/2 cup of pasta water before draining.
In a large skillet over medium heat, heat 3 tablespoons olive oil.
Add chopped onion and cook for about 3 minutes, until softened.
Add garlic, chopped spinach, dried oregano, ground nutmeg, red pepper flakes (if using), salt, and black pepper.
Stir in lemon zest and lemon juice.
Cook for 3 to 4 minutes, until the spinach is wilted.
In a small bowl, whisk together Greek yogurt, heavy cream, and cornstarch until smooth.
Add the mixture to the spinach pan and stir to combine.
Add cooked pasta to the skillet and toss to coat.
Add a splash of reserved pasta water as needed to loosen the sauce and create a silky texture.
In a small skillet over medium heat, add olive oil.
Stir in panko bread crumbs, dried oregano, and salt.
Toast for 3 to 4 minutes, stirring frequently, until golden brown and crispy. Remove from heat.
Plate the pasta and top with crumbled feta cheese, grated Parmesan cheese, chopped fresh parsley, and toasted breadcrumbs. Serve hot.