Cook the pasta according to package directions, reserving 3 cups of the cooking water. Drain in a colander and set aside. Return the empty pot to the stove.
Set the heat to medium. Into the pot used for cooking the pasta, add 4 tablespoons of butter. When the butter is melted, add the onion. Cook for 2 minutes, stirring frequently until softened. Add the garlic and continue cooking for an additional 1 minute.
Add the spinach leaves. Cook for 2 minutes, stirring occasionally until the spinach is wilted. Stir in the lemon zest, dill, ground black pepper, 1 cup of vegan feta cheese, and 2 cups of the reserved pasta water. Stir to coat and continue cooking for 3 minutes. The sauce will begin to thicken as it heats. If it gets too thick, just thin it out again with a little more pasta water.
Remove from heat and stir in the remaining 4 tablespoons butter and 2 tablespoons lemon juice.
Top with the remaining 1 cup feta. Adjust seasoning to taste and serve hot.