Start by chopping & cleaning the vegetables. Remove the dark green leaves from the top of the leeks and set aside, slice each leek in half and run them under cold water.*
Slice the zucchini in half and slice into one-inch pieces. Chop the potatoes into small one-inch pieces - this will help them cook quickly and evenly.
In a large stockpot over medium heat, add the butter & olive oil. Add in all of the vegetables and season with pepper & red pepper flakes.
Sauté the veggies, stirring occasionally for about 5 minutes or until they just begin to soften.
Add in about 3 cups of stock - just enough to cover the vegetables. Increase the heat to medium-high and bring to a boil.
Continue to boil until the potatoes are fork tender, turn off the heat. Add the parmesan cheese and stir until melted.
Ladle into a blender in batches or use an immersion blender and process until smooth and creamy.
Taste to adjust the seasoning. It may need salt, but because the parmesan is quite salty perhaps not - just taste and add if needed.
Top with shredded leaves of fresh basil if desired, enjoy!