1-Cut tomatoes in quarters and puree them in food processor along with bell pepper. Strain puree through a coarse sieve to remove skins and seeds. (You can dump the puree into a colander and work it through with your hands until there is nothing left in the colander but a dryish pulp of skins and seeds.)
2-Now puree onions, combine with tomato and pepper puree, and pour into a large stainless steel or enameled kettle. Cook and stir occasionally over low heat until it is reduced by about a third and is considerably thicker.
3-Meanwhile put garlic, peppercorns, allspice, cloves, cinnamon, and celery seed into the vinegar in a small pot and simmer covered for 1/2 hour to steep spices in the vinegar.
Pour about half the spiced vinegar through a tea strainer into the thickened tomato mixture. Stir. 4-Also add sugar, mustard, cayenne, and salt at this point. Here is where the tasting comes in.
You can adjust any of these ingredients to suit you. You can add more spiced vinegar. Or a little plain vinegar. More or less sugar, mustard, cayenne.
5-Just sort of tinker with it. Cook it some more, stirring often, until it looks like catsup should look. Taste and adjust again. You may notice that it looks slightly curdled. Not to worry. Hit it a lick in the food processor. Smooths right out.
6-Pour into sterile jars leaving 1/8"" of head space. Process in a boiling water soak 15 minutes.