It's important to wash leeks thoroughly as dirt likes to wiggle in between their layers. To clean them, cut the bottom (where the roots are) off, as well as the darker green leaves. Halve the white and light green part lengthwise. While rinsing, use your fingers to create space for water to get in between the layers. Pat to dry.
Pro bacon tip: Leeks love bacon. If you're not the biggest fan of mushrooms, swap them out for bacon or pancetta. Just take them out of the pot once they've cooked, and stir them back in with the parmesan so they stay crispy.