In a large braising pot, one that will fit the peppers snugly and that has a lid, add the oil and heat over low heat.
Add the garlic, sauté for less than 10 seconds.
Add the tomato paste, careful, it may splatter.
Add the stock, whisk to combine the tomato paste with the stock.
Add the sugar, salt and pepper. Bring to a boil and let simmer for a few minutes. Remove about ¾ of a cup of sauce, set aside.
In the meantime, stuff the peppers with the filling ⅔ full. Place in the sauce (cut side up), squeezing the peppers in, the less space the better. If you have any space in between the peppers, fill it with the extra whole (or half) peeled onions (it will flavour the sauce).
Pour the reserved ¾ cup sauce over the peppers with filling.Put the lid on the pot, turn heat to low and let simmer for 45 min-1 hour.