Bring a large pot of salted water to a boil. Cook the pasta according to package directions until al dente. Drain and set aside. No need to rinse.

Heat 2 tablespoons of olive oil in a large skillet over medium-high heat. Add the sausage and cook, breaking it up with a spoon, until browned on the outside, about 5-7 minutes. Transfer to a plate and set aside.

In the same skillet, add the butter and remaining olive oil. Once hot and shimmering, add the mushrooms and shallots. Season with a pinch of salt and pepper. Let them cook undisturbed for 5 minutes to develop color, then stir and continue cooking another 3-5 minutes, until the mushrooms are tender.

Add the garlic and oregano, and cook for about 30 seconds, just until fragrant. Sprinkle in the flour and stir constantly for 1 minute to create a light roux.

Slowly whisk in the milk and chicken broth. Season again with salt and pepper. Bring to a gentle simmer, then return the sausage to the pan. Cook 3-4 minutes, stirring occasionally, until the sauce thickens slightly and the sausage is cooked through.

Stir in the pesto, then fold in the spinach. Cook just until the spinach wilts, about 1-2 minutes.

Add the cooked pasta and toss until everything is well coated in the sauce. Serve warm, garnished with fresh parsley or basil if you’d like.