For a handful of weeks each summer, I go into full-on raw-tomato mode. When they're at their peak, the less you do to them, the better (see: Caprese salad, tomato bruschetta, and panzanella). But during the rest of the year, tomatoes need some help.
This is true of both canned whole tomatoes and fresh ones. Usually, that help comes in the form of cooking the tomatoes, which concentrates their flavor and sweetness by driving off excess moisture—and, in the case of canned tomatoes, gets rid of some of that straight-from-the-can flavor. If you're making a sauce, it's easy to do: Just crush or purée the tomatoes, then simmer them down until you get the flavor you want. But that won't really work if you want to keep the tomatoes chunky and at least a little firm, as a mix-in for salads or a topping for a sandwich.
At times like that, it's best to turn on your oven. Whether they're canned or fresh, by increasing the surface area of the tomatoes on a baking sheet and using low, dry heat to drive off their moisture, you can concentrate their flavor and preserve their structural integrity. It's a trick I use in the wintertime to make a tomato topping for bruschetta that's just exploding with flavor, but roasted tomatoes are wonderful for so much more. Add them to a pan sauce, mayonnaise, or vinaigrette for a boost in texture, color, and flavor; toss them with a potful of clams or mussels to produce a more complex broth; or mix them with roasted vegetables, or even grain or bean salads, as an unexpected but very welcome interloper. Here's how.
If using canned whole peeled tomatoes, drain them to start, then tear each tomato open with your fingers to remove and discard the seeds. This speeds up the roasting process; otherwise, the sheer amount of time required to dry out the tomatoes would outweigh the resulting flavor boost.
Arrange the tomatoes on a parchment-lined rimmed baking sheet, drizzle them with olive oil (and maybe scatter about a few sprigs of thyme or rosemary, if you have them), and pop them in a 300°F oven. Roast until their excess juices have evaporated and the tomatoes look slightly dried on the outside but still moist within, about an hour. After that, I usually chop them up.
With fresh but out-of-season tomatoes, the only difference is that I'll sometimes remove the skins from the tomatoes before halving and seeding them, which you can do by following the directions here. You can also leave the fresh seeds in and the skin on, which will produce a juicier result that works well if you plan on leaving them whole. Plum tomatoes are especially good for this technique, since they have a higher proportion of flesh to seeds.
In one restaurant where I used to work, we'd oven-dry fresh plum tomatoes and then layer them in a light spring lasagna. They'd be equally at home on a sandwich (and way better than mealy fresh ones), or as a non-sauce tomato topping for pizza.
Since they're not cooked beforehand, the way canned tomatoes are, fresh tomatoes hold their shape even better after slow-roasting, but they won't have quite as intense of a flavor. Fresh and canned are largely interchangeable here, but if you have a choice, canned are preferable if you're going to chop them up, while fresh perform better when left whole or in larger or more distinct pieces.
So the next time you find yourself with less-than-ideal tomatoes, don't fret—you're just an oven away from a batch of rich, intensely flavored tomatoes that can transform any meal.