Butter and cream usually mean mashed potatoes are off-limits for vegans, but this variation gets its silky texture from white beans instead. Adding the legumes also doubles the amount of protein compared with traditional spuds.
Heat oil in a medium saucepan over medium heat. Cook onion, celery, and carrot until translucent, 6 to 8 minutes. Add potatoes and white beans and cover with water by 2 inches. Season generously with salt. Bring to a boil, then reduce heat and simmer until potatoes are tender, about 8 minutes. Drain, reserving about 1 cup cooking water.
Mash vegetables, adding reserved water to adjust consistency. Season with salt and pepper and drizzle with oil before serving.
For an even smoother mash, press the cooked vegetables through a potato ricer.