Preheat the broiler. Put the quartered tomatoes, sliced onion, and whole garlic cloves onto a roasting tray, spreading out evenly. Drizzle with plenty of olive oil and season well with salt and pepper and sprinkle with cilantro sprigs. Broil until everything is nicely charred, about 10 minutes (you want lots of deep rich color so don't be afraid if some of the edges get pretty black).
Dried chilies do not offer the same freshness, fruitiness and immediate punch that fresh chilies do. On the other hand, though, they have a complexity and depth of flavor that fresh chilies simply do not possess: A raisiny, dried fruit sort of quality with a hint of smoked earthiness and a slow, lasting warmth rather than an initial burst of heat. In applications such as Salsa Roja, I much prefer this added depth and complexity as well as the consistency from batch to batch achieved by using a dried product.
As you will notice when we dive a little further into these recipes, Mexican food has a stunning range of somewhat special ingredients. From Masa Harina, the essential ingredient of corn over a wide array of chilies either fresh, dried or ground to Mexican oregano and tomatillos. Some of these may at first glance seem foreign and difficult to come by, but fear not we’ll discuss the individual ingredients as we go and they’ll be easier to come by than you think.