Preheat oven to 325F. Toast bread in hot oven to golden brown. Pulse the bread in food processor to make fresh, large breadcrumbs.
Using the tip of a very sharp paring knife, remove the core of the Brussels sprouts and separate leaves. You should yield 3 cups loosely packed leaves.
Bring a large pot of water to a boil. Salt water.
Peel the potato and cut into thirds horizontally, then slice the thirds of potato across lengthwise, into about 1/4-inch slices. Boil potatoes 5 minutes, add Brussels sprouts leaves after 2 minutes. Remove potatoes and cabbage from water with a spider or strainer and transfer to a serving bowl.
Cook pasta in boiling water to al dente in the same water. Reserve a little starchy cooking water just before draining.
Meanwhile, heat butter, garlic and sage in small pot over low heat, do not brown.
Add pasta to potatoes and cabbage in the serving bowl. Pour butter and sage over top and season with salt, pepper and nutmeg. Add some cheese a couple of handfuls, toss pasta to combine and taste to adjust seasonings. Use a splash of starchy cooking water if pasta gets too tight while tossing.
Serve spaghetti in shallow bowls topped with more cheese and the breadcrumbs.