Heat 1 tablespoon of the olive oil in a wide saute pan over medium heat. Add half the mushrooms, season with a few pinches of salt and pepper, and saute until they’re well colored, 4 to 5 minutes. Scrape the mushrooms onto a plate, add the remaining oil to the pan, and cook the second half of the mushrooms. Combine the mushrooms in the pan, add most of the scallions and the garlic, and cook for another minute. Then pour in the stock and add the beans. Simmer until the mushrooms are tender, about 10 minutes, then turn off the heat and set aside.