Puree is the starting point for homemade ketchup, relish, sauce, chutney - any thick tomato-based sauce. Make tomato puree by cooking washed, quartered ripe tomatoes gently in a large stock pan over medium-low heat until they become very soft. Stir often to prevent sticking.
This is a slow food process; do not rush. It may take 90 minutes for a large stock pot of tomatoes to melt down. Run them through your Foley Food Mill, an indispensable, indestructible, inexpensive food preservation tool.
Return the juice to your large stock pot and continue cooking slowly until at least a third of the liquid evaporates, thickening your tomato juice into puree. You can also cook down juice in a large stainless steel dishpan in a 300 degree oven, stirring every 20 minutes.
The amount of time required to go from juice (delicious in itself) to puree (good base for chutney, sauce, ketchup, relish) depends on your personal preference and, more, on the kind of tomatoes you use. Paste or plum tomatoes are meaty and less juicy than typical salad tomatoes. Paste tomatoes will thicken into puree much more quickly than juicy types.