Heat the oil in a frying pan, and add the finely diced onion, mushrooms and garlic. Cook over a medium-low heat for at least 5 minutes, until soft and fragrant.
Push the vegetables to the outside of the pan, and add the butter to the middle. Allow it to melt, then add the flour, and mix to combine. Add the Marmite, and mix well, incorporating the vegetables back into the mixture.
Add the hot water (or any leftover water you might have from cooking vegetables - as long as it's unsalted), honey, sage, and plenty of black pepper. Mix well, and bring to a simmer. Cook for at least 5 minutes, or however long it takes to reach your desired consistency (I like mine fairly thick).