Melt the butter over medium heat until it becomes frothy.

Add the chopped vegetables (known as mirepoix) and sauté for a few minutes until lightly browned. Gradually stir in flour until it is fully incorporated and forms a thick paste, called a roux. Lower the heat and cook the roux for another 5 minutes or so, until light brown. The roux will have a slightly nutty aroma at this point.

Slowly add the stock and tomato purée to the roux, whisking vigorously to make sure it is free of lumps. Bring to a boil, lower the heat and add the bay leaf, thyme and parsley, cooking for about 50 minutes or until the sauce has reduced by about one-third, stirring often so it does not burn.

Remove the sauce from the heat, and carefully pour the sauce through a medium sieve lined with a piece of cheesecloth or fine sieve. This will remove any lumps and retrieve the bay leaf, parsley and vegetables.