Roughly chop the onion.
Heat 1 tbsp of oil in a medium-sized non-stick saucepan on high heat.
Add the onion, reduce the heat to medium, and cook for 3 to 5 minutes, stirring regularly, until the onions start to turn golden.
Mix the stock, tamari, garlic powder, nutritional yeast and black pepper in a large jug, then pour into the pan.
Bring to the boil, then reduce the heat to a gentle simmer.
Remove 5 tablespoons of the sauce to a jug and sift in the thickening agent, whisking well so it is fully incorporated, and add back into the pan.
Add the olive oil to the pan and simmer for 3-4 minutes until it thickens, whisking occasionally.
Sieve out the onions before serving with your Christmas Wellington or other festive favourites!