Halve the Hokkaido, remove the stones and cut the pulp into large pieces. Peel the sweet potato, first into slices, then into large pieces. Peel the asparagus and cut off the woody ends, cut the asparagus stalks into pieces, put the asparagus heads aside, peel the onion, cut in half and quarter. Heat 1 tablespoon of oil in a saucepan and sauté the vegetables in it.