Preheat oven to 400ºF.

Cook the pasta:

• Bring a large pot of salted water to a boil. Cook the pasta according to package instructions and reserve 1/2 cup pasta water. Drain and set the pasta aside. Note: Try to time this so that the pasta finishes cooking right as the sauce is done.

Roast the squash:

• Arrange the squash slices on a baking sheet and drizzle with 1 tablespoon oil. Season with salt and pepper. Roast for 20–25 minutes until golden-brown all over. Turn off the oven and keep the squash warm.

Start the sauce:

• Heat 1 tablespoon extra virgin olive oil in a wide pot over medium heat. Add the shallot and cook for 5 minutes, often stirring, until softened.
• Add the celery leaves or garlic and cook for 45 seconds until fragrant—season with salt.

Prepare the roux:

• Add the remaining 1 tablespoon extra virgin olive oil (or use 1 tablespoon butter). Add the flour and cook, sizzling it in the oil and stirring until nutty and aromatic.
• Add in half a cup of milk and constantly whisk until a slurry forms.
• Continue adding milk in half cup increments until all of the milk is incorporated. Whisk after each addition. Bring to a very low boil and then simmer for 15 minutes, occasionally stirring, until the sauce is thickened to your liking—taste and season with salt and pepper.

Finish the sauce:

• Add the arugula and cook until wilted. Add the parsley and lemon juice and cook for 1 minute more—taste and season once more to your preferences. Reduce heat to low.
• Sauce the pasta:
• Add 1/4 cup of the pasta cooking water along with the cooked pasta and toss to coat. If needed, add a splash of the pasta water until the sauce is glossy and completely coats the pasta. Turn off the heat.

To serve:

• Divide the pasta between plates and arrange the roasted squash on top. Garnish with more parsley, if you like. Enjoy!