In a large pot, cook the pork belly over low heat until the fat has melted.
Add the minced garlic, mirepoix and cook over medium heat, until soft and golden, 5 minutes.
Add the flour (singer) and cook for 3 minutes. In this step, you will make an roux using a previously added pork fat.
Stir in the tomato paste and let it cook, 2 minutes.
Add the tomatoes, salt and sugar, mixing well. Remember, the sugar is to soften the acidity of the tomatoes and not to sweeten the sauce, which should have a balanced taste, neither acidic nor sweet.
Add the tomatoes, chicken stock, bouquet garni, pork belly rind, bring to a boil, cover with lid, and transfer to the oven. Simmer for 2 hours.
Remove from the oven and discard the rind and bouquet garni. Let cool slightly, and purée in a blender or food processor until smooth.
Season with salt and pepper to taste. Strain through a sieve.
Use immediately, keep in the fridge for 5 days, or freeze for up to 6 months.