Melt the butter in a large saucepan over medium heat. Add the onion, garlic and 1/2 teaspoon salt and cook until the onion is soft, about 2 minutes. Add the oat fiber and cook, stirring, until lightly toasted, about 1 minute. Whisk in the nut milk and cream and cook, whisking constantly, until thickened, about 1 minute. Remove from the heat. Stir in the lemon juice, Worcestershire sauce, parmesan, nutritional yeast and sour cream.