Edit: It has been quite rightfully brought to my attention that I should mention that pan con tomate in its original for is actually a Catalonian dish, not Spanish, and that in fact, pa amb tomàquet is a more geo-culinarily appropriate name for the dish. Here's to the Catalan and their incredible contributions to the food world!
Pan con tomate is just about as humble as tapas can get. It's got only five ingredients—bread, tomato, olive oil, garlic, and salt—and requires barely any actual cooking, yet it's precisely this simplicity and restraint that make it such a perfect end-of-summer dish. This is the kind of thing I make as an appetizer at a party or combine with a hearty salad for a light dinner. It takes just minutes, and, more than anything, it highlights the quality of your ingredients. This happens to be a good year for tomatoes, which makes it a good year for pan con tomate.
As with a good Caprese salad, the only real way to totally mess up pan con tomate is either to start with subpar ingredients (you want the best tomatoes, olive oil, and bread) or to overthink it. This is one of the few cases in which the lazier you are, the better.
Unlike its Italian counterpart, tomato bruschetta—another dish made with the same basic ingredients—Spanish pan con tomate uses tomato pulp, not diced or sliced tomatoes. How you get that pulp can vary.
Some folks like to keep it super simple by splitting a tomato in half and rubbing it over the rough surface of a slice of toast, tinting it red and giving it a very light, refreshing tomato flavor. Tomato as condiment. This is a fine method if you've got lots of bread and not many tomatoes, but that's not typically my situation (good tomatoes tend to attack in packs).
I use the box grater method, a technique I learned while working at Toro, one of Ken Oringer's two Spanish restaurants by the same name in Boston and New York: Cut a tomato in half and rub the cut surface over a box grater, keeping the palm of your hand completely flat. This conveniently extracts and chops the tomato pulp, completely separating it from the skin, which you can then discard. (It's also a great method if you want to make a super-quick fresh tomato sauce or puree to use on pizzas or pasta.)
The bread is typically thin slices of long, rustic loaves, which work just fine if you're serving pan con tomate as you often do in Spain: as a free bar snack. But for a more substantial part of a meal, I like to use more substantial bread, something I can really pile the tomatoes onto.
A loaf of open-holed ciabatta, split in half lengthwise and then split crosswise into individual pieces, provides enough substance and structure for big spoonfuls of tomato pulp. I drizzle extra-virgin olive oil over the bread and place it under the broiler until it's browned, crisp, and just starting to char right around the edges. You need that level of crispness to stand up to the juicy tomato.
The final question is how to incorporate the olive oil and garlic. Some recipes, like this one from Tertulia's Seamus Mullen, call for adding the garlic directly into the tomato pulp, along with some sherry vinegar.
I prefer to keep my flavors a little more distinct, letting those tomatoes handle themselves. The garlic I apply directly to the bread, splitting a clove in half and rubbing it on the rough surface. It acts kind of like sandpaper—you'll see your garlic clove getting smaller and smaller as it leaves an invisibly thin layer of flavor behind.
The tomatoes I simply season with salt and spoon on top of the bread. Finally, I drizzle it with more olive oil (a lot more olive oil), sprinkle it with coarse salt, and serve it. That's it. No embellishments required. You don't even need chopped herbs here, though a sprinkle of parsley or chives would not detract from the overall experience, I suppose. I've seen a few variations here and there with other toppings, but to my mind, the only one that has ever really come close to matching the synergy of flavors of the original is adding a single brined anchovy, along with a tiny dollop of allioli, the Spanish version of a garlicky mayonnaise.
It's only as I write this that I realize why I like that flavor combination so much: Between the toasted bread, the fresh tomato, the mayo, and the salty/savory anchovy, it's got almost all the same notes as the BLT, the best simple sandwich ever conceived.
Should I put shredded iceberg lettuce on my pan con tomate? Should I? I probably shouldn't...but perhaps just a little. In the name of science.