Bring a large pot of salted water to a boil and cook the spaghetti until just al dente, about 8-10 minutes. Drain, reserving a small splash of pasta water, and set aside.
While the pasta cooks, rinse and thinly slice the leeks, ensuring any grit is washed away. Slice the mushrooms into bite-sized pieces and mince the garlic finely.
Heat a large skillet over medium heat and add the olive oil. Once shimmering, add the sliced leeks and cook gently, stirring often, until they turn translucent and soft, about 8 minutes. You should smell a sweet aroma as they soften.
Push the leeks to one side of the pan. Add the sliced mushrooms to the cleared space and cook, stirring occasionally, until they develop a deep golden color and smell smoky, about 6-8 minutes. If the pan gets too dry, add a splash of olive oil or reserved pasta water.
Add the minced garlic to the mushrooms and leeks, stirring constantly for about 30 seconds until fragrant. Be careful not to let the garlic burn, which can turn bitter.
Transfer the cooked pasta into the skillet with the leek and mushroom mixture. Toss everything together gently to coat the noodles evenly, adding a splash of reserved pasta water if the sauce seems too thick or dry. Cook for another 1-2 minutes until heated through.
Finish the dish by adding lemon zest or a squeeze of lemon juice to brighten the flavors. Grate fresh Parmesan over the top if using, and season with salt and pepper to taste.
Serve immediately, garnished with extra Parmesan if desired, and enjoy the glossy, fragrant pasta that combines subtle sweetness with earthy richness.
