Bring a large pot of water to a rolling boil over high heat, then add the diced potatoes and pumpkin cubes. Boil for about 15 minutes, until both are tender enough to mash easily with a fork.

Drain the vegetables in a colander and let them cool slightly while excess moisture evaporates. Transfer them to a large bowl and mash or press through a ricer until the mixture is completely smooth, about 5 minutes.

Sprinkle the salt and flour into the mashed vegetables a little at a time, mixing gently with your hands or a wooden spoon just until a soft dough forms. Be careful not to overwork the dough so the gnocchi stay light and tender; this should take no more than 5 minutes.

Lightly flour a work surface and divide the dough into 4 pieces. Roll each piece into a long rope about 3/4 inch thick, dusting with extra flour as needed to prevent sticking. This step takes about 5 to 10 minutes.

Cut each rope into 3/4 inch pieces. Use a gnocchi board or the back of a fork to press ridges into each piece, then place the shaped gnocchi on a floured tray. Allow up to 10 minutes for this step.

Bring another large pot of salted water to a gentle boil. Add the gnocchi in small batches and cook until they float to the surface, about 2 to 3 minutes. Remove them with a slotted spoon and drain briefly.

Melt the vegan butter in a wide skillet over medium heat and stir in the sage. Add the drained gnocchi and toss gently for 2 to 3 minutes, until they are lightly golden and well coated in the sage butter.