Preheat the oven to 375F. Line a roasting pan with parchment paper and coat it evenly with about 1 tablespoon of the oil (pump sprays work well too).
Coat a cast-iron or sauté pan with 2 tablespoons of the oil and heat over medium-high heat. Add the mushrooms and shallots and sauté., stirring attentively, for 10 to 15 minutes. When the mushrooms and shallots start to caramelize, add the sherry, and continue to stir. Let the sherry cook off and continue to stir until you have a deep, rich-looking pan of mushrooms. Remove from the heat and set aside.
Evenly spread the onion, butternut squash, potatoes, and celery in the roasting pan. Cook for 20 minutes, then remove the pan and flip the vegetables. Place the bouquet garni on top of the vegetables and return them to the oven for 10 to 15 minutes.
When the vegetables are done, combine the mushroom mixture, roasted vegetables, bouquet garni, pesto, butternut squash puree, beans, tomatoes, and 4 cups of broth in a large stockpot. Bring the soup to a low boil and cook for 25 minutes. Add the buckwheat groats and cook for an additional 10 minutes. If you would like to thin out the stew, add in the remaining 1 cup vegetable broth. Remove the bouquet garni, season with salt and pepper to taste, and serve immediately.
NOTES: If you don’t have butternut squash, any 16-ounce bag of frozen root vegetables will work. This is a good opportunity to clean out your freezer!
If you don’t have fresh herbs, substitute dried herbs. Typically, the rule is 3 parts fresh to 1 part dried.