Chop the pumpkin. If you're unsure how, learn from Master Lu in this video. Though unlike the video, leave the skin on: Eating For Healing Kabocha Squash

Steam the pumpkin. Watch for when it's soft and when you can put a fork in. You may need to steam the vegetable in two batches, if you have a small steamer.

Warm the oil with half the salt in a large pan. Add the onions.

Caramelize the onions. Sauté to soften them, and then raise the heat to a loud sizzle. Cover the pan, lower the heat, and stir every now and then until they brown. Add a tablespoon of water if they start to stick. 

Once the onions are browned, add the steamed pumpkin to the pan. Stir in the butter and the mushroom seasoning and salt.

Serve while it's warm.