Begin by bringing a large pot of salted water to a boil. Add the spaghetti and cook according to package directions, typically around 8-10 minutes, until al dente. Reserve 3 cups of the starchy pasta water, then drain the spaghetti and set it aside, keeping it warm in the pot.

In the same pot, melt 4 tablespoons of butter (or dairy-free butter for a vegan option) over medium heat. Once melted, add the chopped yellow onion and sauté for about 2 minutes until translucent. Stir in 2 cloves of minced garlic and cook for an additional minute.

Add the baby spinach leaves to the pot, cooking until they are wilted, about 2 minutes. Then, stir in the lemon zest, dried dill, and ground black pepper. Pour in 1 cup of the reserved pasta water along with 1 cup of crumbled feta cheese.

Remove the pot from heat and stir in the remaining 2 tablespoons of butter and the juice of half a lemon. Mix thoroughly.

Finally, add the cooked spaghetti back into the pot with the sauce, tossing gently to combine. Serve hot, garnished with extra lemon zest or a sprinkle of fresh dill.