Gently cook the veg, herbs and butter in a pan for 10-12 mins until the vegetables start to brown. Scatter over the sugar and continue to cook for 1-2 mins until a little caramelised. Stir in the flour and combine well. Add the Marmite, if using, tomato purée and vinegar. Pour over the stock a little at a time stirring as you go.
Simmer everything together until you have a thickened sauce, this will take about 30 mins. Sieve, then add soy sauce and a touch more Marmite to season and colour, if you like. Will freeze for up to two months.