Bring a large pot of salted water to a rolling boil. Add the spaghetti and cook according to package instructions until al dente, usually around 8–10 minutes. Reserve about 3 cups of the pasta cooking water before draining the spaghetti in a colander.

In the same pot, melt 4 tablespoons of butter over medium heat, allowing it to foam. Add the chopped yellow onion and sauté for about 2 minutes until it softens and becomes translucent. Stir in the minced garlic and cook for an additional minute, until fragrant and lightly golden.

Toss in the baby spinach leaves and stir them into the onion mixture. Cook the spinach for approximately 2 minutes, until it’s wilted and bright green. Then, add in the lemon zest, dried dill, and ground black pepper. Mix well before incorporating the vegan feta (or regular feta), and pour in 2 cups of the reserved pasta water.

Remove the pot from heat, and quickly stir in the remaining 4 tablespoons of butter, allowing it to melt into the sauce for a creamy texture. Squeeze in the fresh lemon juice to brighten the flavors.

Gently add the drained spaghetti back into the pot, tossing until the pasta is completely coated with the delightful sauce. Adjust seasoning as needed, and serve hot, garnished with additional feta and a sprinkle of fresh dill.