In a medium saucepan, heat oil to shimmering over medium heat. Reduce heat to and add minced vegetables. Cook gently, stirring occasionally until vegetables are transparent and begin to soften.
Add flour, stir using a whisk or wooden spoon until the flour is pale golden brown but does not burn.
Whisk in tomato paste, choppped mushroom, stock or broth and bouquet garni, salt and pepper. Bring to a boil, whisking constantly. Then partially cover pot, reduce heat and gently simmer for 40-45 minutes.
Strain stock using a fine mesh sieve, pressing vegetables with a spoon to extract as much liquid as possible.
Return sauce to clean pan and continue simmering sauce until it is thick and glossy. It should coat the back of a spoon.
Use sauce immediately, or allow to cool and transfer to a jar and store in the refrigerator or freeze.