Preheat oven to 400 °F.
Place a large pot of water over high heat and bring up to a boil to cook the pasta. Once at a boil, add some salt and the pasta and cook until al dente, according to package directions. Drain pasta thoroughly, return it to the pot and reserve.
In a large skillet over medium-high heat, heat 1 turn of the pan of EVOO and the butter. Cook garlic and onion until tender, about 2-3 minutes. Sprinkle in flour and cook for another minute then whisk in chicken stock and half-and-half, season with salt, ground black pepper and nutmeg. Bring it up to a bubble and thicken.
Turn off the heat, fold in the parsley, grated Parmigiano Reggiano, cannellini beans, spinach, tuna, and penne. Pour the mixture into a 13x9" baking dish and top with remaining cheese. Place in the oven to brown the topping, about 10 minutes.
Serve with your favorite simple salad alongside.