Wash and cut tomatoes into quarters. Roma tomatoes won't need to have juicy insides removed, but if using a juicier variety, pull the jelly inside out with fingers. Add tomatoes to a pot.
Heat cut tomatoes on medium low heat until they begin to break down and become soft. Stir occasionally to avoid sticking. Once they release more moisture the heat can be increased to a medium simmer until tomatoes are reduced by half.
Preheat oven to 275degrees. Lightly rub olive oil over a large, rimmed baking sheet such as a half sheet pan (use parchment paper for aluminum baking sheets).
Work cooked tomatoes through a food mill or mesh strainer to remove skins and seeds. Pour strained tomato pulp onto oiled baking sheet.
Spread tomato pulp evenly over baking sheet so that there is a uniform thickness less than ¼ inch but thicker than ⅛ inch. (Too thin and it will burn.)
Bake at 275 degrees Fahrenheit for 45 minutes to1 hour, using a bench scraper or rubber spatula to scrap up and redistribute tomato pulp evenly every 20-30 minutes (closer intervals towards the end of cooking) until thick and darkened in color. You will notice a reduction in size as the moisture evaporates and it becomes a paste.
Reduce heat to 200 degrees Fahrenheit, and continue to cook, scraping and re-spreading every 60 minutes. This will take anywhere from 2-4 hours for 5-10 pounds of tomatoes.
Cool tomato paste. Divide into 3-6 tablespoon portions by scooping spoonfuls onto a cutting board or spooning into ice cube trays. Freeze solid, then when frozen the portions can be collected into a zip lock freezer bag and stored in the freezer for up to 5 months.