Cook the Spaghetti: Bring a large pot of salted water to a boil. Cook the spaghetti according to package directions until al dente. Drain, reserving 1/2 cup of pasta water, and set aside.

Sauté the Leeks and Garlic: In a large skillet, heat the olive oil over medium heat. Add the sliced leeks and sauté until softened and lightly caramelized, about 5-7 minutes. Add the minced garlic and cook for another 1-2 minutes until fragrant.

Make the Creamy Sauce: Reduce the heat to low and stir in the heavy cream and vegetable broth. Let the mixture simmer for 3-4 minutes until it thickens slightly.

Add Parmesan and Pasta: Stir in the grated Parmesan cheese and reserved pasta water. Mix until the sauce becomes creamy and smooth.

Toss the Spaghetti: Add the cooked spaghetti to the skillet and toss it in the creamy sauce until evenly coated.

Season and Serve: Season with salt and pepper to taste. If desired, squeeze fresh lemon juice over the pasta to add brightness. Garnish with chopped parsley before serving.