Melt 3 tablespoons of the unsalted butter in the same Dutch oven over medium heat. Add the onion and cook, stirring occasionally, until softened, about 5 minutes. Add the garlic, 1 pound sliced cremini mushrooms, and 1 teaspoon of the kosher salt. Cook, stirring occasionally, until the mushrooms and onions start to brown, 8 to 10 minutes. Meanwhile, bring a large pot of salted water to a boil.