This is a pantry basic for me. I fire-roast my tomatoes and drain off liquid (see my post on passata) from them to make it. It certainly cuts down the boiling-down time to produce tomato sauce! I usually produce 1 pint of tomato stock for every 2 pints of tomato sauce (this is still some boiling down) I put up.
You can refrigerate the drained liquid for several days if you're accumlating it from several small batches of sauce making.
Recipe for Tomato Stock
12 cups of drained tomato liquid
2-3 cloves of garlic
1/2 c fresh basil
1 small onion, cut up
1-2 stalks of celery, cut up
1/2 c parsley
other herbs to taste: tarragon, hot pepper, oregano
Bottled lemon juice or citric acid for jars
Put 1 cup of the liquid in the blender. Add the seasoning ingredients. Puree.
Put the rest of the liquid into a pot. Add the seasoning puree. Salt to taste. Prepare your jars as you heat the stock to boiling; you can simmer it for 5 to 10 to develop flavours for your tasting and adjusting.
For each pint jar, use one tablespoon of lemon juice or 1/4 teaspoon of citric acid for acidification (double amount for quart jars). Pour stock into jars, seal, and process 35 minutes for pints, 40 minutes for quarts.