Tear up the bread and put it into a small bowl. Pour over the milk and leave to soak while you measure out everything else.
Put the walnuts, garlic, grated parmesan and olive oil into a small blender or food processor. Squeeze out the excess milk from the bread (but don't discard yet) and add it in as well. Blend them all together until well combined and smooth.
Add a little of the reserved milk if needed to help make the sauce relatively smooth and not too thick. Add a little salt and pepper, to taste. Set aside until pasta is cooked.
When your pasta is ready, don't discard all the pasta cooking water - keep some so you can add a little of the pasta water to thin down the sauce and help it become nice and creamy as you stir the sauce through the hot pasta.
Video
Notes
You can add as much or as little sauce as you like, but this quantity is good for around 1lb/450g pasta, or more.