Bring a large pot of water to a boil and salt it generously.
While the water heats, soak the slice of bread in the milk in a small bowl until soft.
In the bowl of a food processor, combine the walnuts, garlic, Parmesan cheese, and marjoram. Pulse until the mixture is finely ground.
With the food processor running, slowly stream in the extra virgin olive oil until the mixture forms a very thick, smooth paste.
Gently squeeze the excess milk from the soaked bread (reserve the milk!) and add the softened bread to the food processor. Pulse to combine.
With the machine still running, pour in the reserved milk and add a splash of water, if needed, to reach a smooth, saucy consistency.
Season generously with salt and pepper.
Cook the pasta in the boiling water according to package directions until al dente.
Before draining, reserve about 1 cup (240 ml) of the starchy pasta water.
Drain the pasta and return it to the pot.
Add the walnut sauce and toss to coat everything evenly. If the sauce is too thick, add a splash of the reserved pasta water to thin it to your desired consistency.
Taste and adjust seasoning with more salt and pepper if needed.
Serve immediately, topped with a sprinkle of extra Parmesan cheese.
