Prep the vegetablesThinly slice the onion. Cut the kabocha squash in half and remove the seeds and stringy pulp using a spoon. Cut the kabocha into 1-inch pieces and remove the green skin.Tip: Raw kabocha is very hard. If it’s difficult to cut, wrap the kabocha in plastic wrap and microwave for 3–4 minutes to soften it slightly before cutting.
Sauté the onion and make a rouxIn a pot over medium heat, add the olive oil and butter. Once melted, add the sliced onion and sauté until soft and translucent. Sprinkle in the flour and stir continuously until the flour is fully incorporated and no lumps remain.
Cook the kabochaGradually add the soup broth while stirring, then add the cut kabocha. Bring to a gentle simmer and cook for 5–7 minutes, or until the kabocha is tender and can be easily pierced with a fork.
Add milk and blendPour in the milk and warm for 2–3 minutes (do not boil). Remove the pot from heat. Use an immersion blender, or transfer to a stand blender, and puree until the soup is completely smooth.
Finish and seasonReturn the soup to low heat. Add the heavy cream, salt, and sugar, then stir well. Taste and adjust seasoning—if the kabocha is naturally sweet, you can skip the sugar.
ServeServe warm, garnished with chopped parsley if desired.