Bring a large pot of salted water to a rolling boil and cook the pasta until just shy of al dente, about 1-2 minutes less than package instructions. Reserve 1 cup of starchy pasta water, then drain the pasta.
While the pasta cooks, heat a large deep skillet over medium heat. Add 1 tablespoon of olive oil and swirl to coat. Once shimmering, add the sliced garlic and cook until fragrant and golden, about 30 seconds, stirring constantly to prevent burning.
Add the fresh spinach to the skillet in batches if needed, stirring until each handful wilts and becomes bright green, about 2 minutes. If it sticks, splash in a little pasta water or olive oil.
Remove the skillet from heat once the spinach is wilted and bright. Return the drained pasta to the pot or skillet, off heat, and toss to combine.
Gradually add about half of the reserved pasta water to loosen the mixture, stirring gently to create a light sauce that coats the pasta and spinach evenly.
Crumble the feta cheese directly over the hot pasta and spinach, stirring gently so the feta melts into creamy ribbons, about 1 minute. The mixture should look saucy and inviting.
Squeeze fresh lemon juice over the pasta, then season generously with salt and freshly cracked black pepper, tasting and adjusting as needed for brightness and seasoning.
Finish by drizzling the remaining olive oil over the dish, tossing gently to combine all flavors. Serve immediately while warm, garnished with extra feta if desired.
