Heat a large pot on medium high. Add the butter and 2 tablespoons of olive oil and when the butter has melted, add the onions and cook for 3-4 minutes until softened.
Throw in the garlic and thyme and stir. Then add the mushrooms, season with 1 teaspoon of sea salt and some black pepper. Stir for 7-8 minutes until the mushrooms are tender and most of the liquid has cooked off. Add the tamari or soy sauce and vinegar to help deglaze the pan.
Stir in the oats, ensuring every grain is coated. Add 1/2 cup of stock and stir. Leave to cook, stirring occasionally, until the liquid has been absorbed, before adding your next ½ cup of stock. Continue adding stock, 1/2 cup at a time, stirring well after each addition of liquid, until the grains are tender (you may not need all of the stock). Remove from the heat.
Ladle the porridge into bowls and finish with a drizzle of olive oil and your choice of toppings.