In an uncovered instant pot, combine 3/4 of the chopped celery, chopped onion, vegan butter and 2 tablespoons miso, garlic, and bay leaves and saute on high heat for about five minutes. Add the broth, cover, and cook on low pressure for 5 minutes.
While the stock is cooking, in a heavy sauce pan, heat the remaining butter, miso paste, celery, and pepper over medium high heat unti the celery is soft. Reduce heat to low and cover.
When the stock is finished cooking, release pressure. Transfer to a blendar and pureé for about 30 seconds. Set aside.
Return to the saucepan, increase heat back to medium-high and add the soymilk and nutritional yeast. Bring to a gentle boil, stirring frequently. Pour the purréd soup into the pot and bring to a boil.
Serve immediately or store in an airtight container in the refrigerator for up to five days.