Boil a large pot of salted water and cook the spaghetti according to package instructions until al dente, about 8-10 minutes. Reserve 3 cups of pasta water before draining.

Melt 4 tablespoons of butter in the same pot, add chopped onion, and sauté for about 2 minutes until translucent. Stir in minced garlic and cook for another minute.

Add baby spinach, stirring for about 2 minutes until wilted. Mix in lemon zest, dried dill, and ground black pepper. Add 1 cup of crumbled feta and 2 cups of reserved pasta water, then simmer for 3 minutes.

Remove from heat, stir in remaining butter and lemon juice until melted. Adjust consistency with reserved cooking water if too thick.

Fold cooked spaghetti into the creamy spinach sauce, ensuring even coating. Top with remaining feta and serve hot.