OK, I'm going to keep this short, because it's the easiest thing in the world. Preheat your oven to 200 degrees. Slice your tomatoes about a ¼-inch thick. Lay them out, on one layer, on a tray, salt them lightly and drizzle with a tiny bit of olive oil.
Pop them into the oven and check on them after 2 hours, then every 15 minutes after that. You want them to be dried out, almost like fruit leather, but they're going to taste great -- they'll spark up anything with a punch of tartness, sweetness and some real umami. It'll make so-so tomatoes great, and good tomatoes amazing. They'll last like this, wrapped up in the fridge, for weeks.
Francis Lam is Features Editor at Gilt Taste, provides color commentary for the Cooking Channel show Food(ography), and tweets at @francis_lam.