To prepare a vegetarian gnocchi with spinach and mushroom butter sauce, you will first need to prepare the spinach. You can buy fresh spinach at your local grocery store, or you can purchase frozen spinach from the frozen food section at most grocery stores. If you don't want to buy spinach, substitute kale, chard or spinach that has been thawed in ice water. To make the vegetable butter, heat 2 tablespoons olive oil over medium-high heat. Once the oil is shimmering, add the garlic and saute until fragrant, about 1 minute. Add the mushrooms and cook, stirring, until they are lightly browned, 1 to 2 minutes. Stir in the butter and salt and pepper to taste. Remove the pan from heat and stir in 1 tablespoon of buttermilk until the sauce is thickened.