1) Melt 30g of the butter in a 7-litre pressure cooker over medium-high heat. Add the onion, garlic, the 1 teaspoon salt, and the pepper to taste and cook, stirring occasionally, until the vegetables soften a bit, about 5 minutes.
2) Add the rice and stir to coat. Stir in the carrots, thyme sprigs, celery root, squash, if desired, chicken stock, and wine.
3) Close the pressure cooker lid and bring the pressure up to high (which can take up to 10 minutes), then reduce the heat to maintain an even pressure for 3 minutes. Remove from the heat and press the cooker's pressure indicator stem until no more steam comes out.
4) Carefully remove the lid--the risotto will look a bit soupy at this point. Stir the mustard greens or spinach into the risotto, then let the mixture sit until the greens wilt, about 2 minutes.
5) Stir in the remaining 30g butter and the 80g cheese. Season to taste with salt and pepper, if you like. Pass additional grated cheese at the table.