Place the dried mushrooms in small pot and cover with water or chicken stock. Bring to a boil, then reduce the heat to low and steep.
Heat a heavy pot over medium to medium-high heat with 2 turns of the pan extra-virgin olive oil. When hot, add the butter and melt into the oil then add portabellas. After 6 minutes, add the onions, carrots, celery, garlic, and bay leaf. Saute until tender, 15 minutes, then separate and add spinach. Season the spinach mixture with salt, pepper, and nutmeg. Stir in the tomato paste and cook 1 minute. Deglaze with wine. Remove the dried mushrooms from the steeping liquid. Chop the mushrooms and stir into the sauce. Add the steeping liquid, reserving the last few spoonfuls as any grit on the mushrooms will have settled at the bottom. Stir in the milk. Simmer over low heat 20 minutes. Cool completely and store for a make-ahead meal.
Reheat the dish over medium heat. Add a little stock or milk to loosen the sauce up again.
Bring a large pot of water to a boil, salt the water and cook the pasta to al dente. Drain the pasta and toss with sauce. Serve in shallow bowls, topped with parsley and Pecorino cheese.