Place the leeks in a large bowl and cover with cold water. Allow them to sit for 5 minutes, agitating occasionally. Pull the leeks out of the water and drain well in a colander.

In a large pot, heat olive oil over medium heat. Once shimmering, add leeks and garlic and cook for about 5 minutes, or until the leeks begin to soften. Add zucchini, Italian seasoning, salt, pepper and red pepper flakes and stir well to incorporate the seasoning. Cook for 8 to 10 minutes, stirring often, or until the zucchini is very soft.

Add chicken stock and beans and bring to a boil. Reduce heat to a simmer and cook for about 15 minutes. Remove from heat and let cool slightly, then transfer to a high speed blender. Add basil, lemon zest and juice and Parmesan (if using). Blend until smooth and serve.