Bring a large pot of water to a boil, salt it and add the spaghetti and the potatoes. Cook until the spaghetti is al dente and the potatoes are just tender, about 10 minutes; drain. (If using fresh buckwheat noodles, cook the potatoes separately in a large saucepan until just tender, and cook the pasta after Step 2, since it only cooks for 2 minutes.)
While the pasta is working, in a large skillet, heat the EVOO, 2 turns of the pan, over medium heat. Stir in the cabbage, onion and garlic; season with salt and pepper. Cook until tender, 8 to 10 minutes. Stir in the vegetable broth and simmer for 5 minutes.
In a medium skillet, melt the butter over medium-low heat. Add the sage leaves and cook until crisp, 3 to 5 minutes.
Toss the pasta and potatoes with the cabbage mixture. Then add the sage leaves and cheese and toss.