Bring a large pot of water to a boil, cook pasta for 8 minutes.
Meanwhile in a saucepan sauté onion with 2 tablespoons olive oil for two minutes, until translucent. Stir in squash, sauté for several minutes, then add veggie broth and simmer until squash is cooked through. Let cool down and carefully process in the blender until smooth.
In a skillet sauté leek with 1 tablespoon olive oil for 3 minutes, stir in squash mixture, bring to a boil.
Drain orzo and add to the veggies, cook for three minutes over high heat, until sauce is creamy and perfectly coating the pasta. Turn off the heat and let rest three minutes. Stir in cheese and butter before serving.
