In a medium Sauce pan, melt the butter over medium heat. Add the onions, and cook stirring often until they are caramelized, about 10 minutes.
Add the flour, and stir constantly while scraping the bottom of the pan. Cook until the mixture is a chocolate brown.
Add the stock a little at a time, incorporating it after each addition. Add the salt, and pepper, and stir constantly until the mixture is well blended, and there are no lumps. Reduce the heat to medium low, and simmer until the gravy has thickened and coats the back of a spoon. Remove from the heat. Taste and adjust seasoning.
Add soy sauce, and kitchen Bouquet as desired, and stir in. Allow to cool before refigerating. You can store this in the fridge for two days, or freeze for up to three months.
For a smoother gravy, puree with an immersion blender or in a food processor.