The best method for sun-drying tomatoes depends on where you live and how much time you have. True sun-drying works well if you can wait at least a week and you live in a dry, hot area that consistently gets full sunlight. If you don't live in a temperate climate, the oven or dehydrator is your next best choice to produce quality dried tomatoes. You first need tomatoes that respond well to drying -- not too juicy but still full of flavor -- plum tomatoes are the best choice. The most common drying tomato variety you find in supermarkets is San Marzano. Fourteen pounds of tomatoes yields about 1/2 pound of dried tomatoes.
Select firm, ripe tomatoes; overripe tomatoes break down during the drying process and practically invite bacteria. Pull off the stem, if present, and rinse the tomatoes under cool water, using your fingers to rub away any residue. Dry the tomatoes with paper towels.
You can slice the tomatoes lengthwise in half or in thirds, but don't slice them thinner than 1/2 inch. Seeding is optional and tedious, as the pulp should remain intact; it's up to you, but it isn't worth the effort. Make a vertical slice through the skin on the tomato halves using a sharp knife to speed drying.
Quick and controllable, oven-drying takes the guesswork out of dehydrating tomatoes, but it has a few shortcomings. Oven-drying requires diligent temperature monitoring -- the oven should stay between 140 and 150 degrees Fahrenheit -- which proves difficult if the oven's lowest setting is Warm, or about 180 to 200 F.
Adjust the oven so the thermometer reads between 140 and 150 F consistently. If you can't get the oven that low, prop the door with a dish towel. Arrange the tomatoes flesh-side-up on a sheet pan lined with parchment paper; the tomatoes can touch but not overlap. Slide the tomatoes on the oven rack; if you have multiple pans, you can stack them perpendicular to each other.
Dry the tomatoes for six to 24 hours. Check the tomatoes and the oven temperature every few hours starting at the 6-hour mark.
Sun-drying doesn't necessarily produce a better sun-dried tomato; dehydrators consistently mimic the environmental conditions that leads to a superior product. You will appreciate the rustic simplicity of halving a few pounds of tomatoes and letting the sun from whence they came essentially reverse the growing process and transform them back to their essence. If you have time and weather on your side, go for it. Select a spot that gets direct sunlight during the hottest part of the day.
Arrange the tomatoes flesh-side-up on a sheet pan lined with parchment. Cover the sheet pan with a mesh screen or cheesecloth. If you use cheesecloth, it cannot touch the tomatoes; they won't receive proper air flow, and they'll stick as they dry. Tent the cheesecloth using a piece of wood placed on each lengthwise side of the pan to keep it above the tomatoes.
Dry the tomatoes for 1 to 2 weeks. Start checking the tomatoes' texture after 1 week. Take the tomatoes inside on dewy mornings.
If you intend to dry tomatoes regularly, invest in a food dehydrator. Dehydrators dry more efficiently than the oven, and you have no concerns over temperature control or environment.
Arrange the tomatoes side by side on the drying trays and slide them in the dehydrator.
Post-Drying Treatment and Storage
Conditioning the tomatoes after drying distributes the residual moisture left in them and increases shelf life. Pack the tomatoes loosely in a large food-grade plastic bag or jar. Seal the jar and let it sit for 10 days. Shake the jar every day to distribute the moisture.
After conditioning, pack the tomatoes in food-grade plastic bags and vacuum-seal them. If you're using sealable food-storage bags, squeeze as much air out as you can before sealing. To pack them in olive oil, pack the tomatoes in a jar, leaving at least 1/2 inch of head space. Add enough olive oil to cover the tomatoes by about 1/4 inch.