Scoop out the seeds and cut the kabocha into large bite-sized chunks, leaving the skin on. Cut the aburaage into strips.
Put the kabocha, beef, and aburaage into the pressure cooker's inner pot, then add the dashi, sugar, soy sauce, and mirin. Lay the kabocha skin-side down.
Set the pot in the cooker, seal, and pressure cook briefly (1-2 minutes is plenty for kabocha). Let the pressure release naturally.
Open the lid and switch to saute/pot mode. Simmer for a few minutes to reduce and concentrate the broth, being gentle so the kabocha keeps its shape.
Spoon into bowls with some of the broth and serve warm or at room temperature.