To Griddle-Dry Chiles: Unless otherwise directed in a recipe, remove and discard the tops and seeds of the chiles. I leave in the veins (the hottest part), but you can cut them away if you want to tone down the heat. Rinse the chiles under cold running water and shake off the excess moisture, but do not dry them.
Heat a griddle or cast-iron skillet over medium-high heat until a drop of water sizzles on contact. Place the chiles on the griddle a few at a time and let them heat, turning occasionally with tongs, just until any clinging moisture is evaporated and the aroma is released. Allow 30 to 45 seconds in all for most chiles (slightly less for guajillos, which are very thin-skinned).
The chiles should just become dry, hot and fragrant; do not allow them to start really roasting or they will have a terrible scorched flavor. Remove from the griddle as they are done. Most recipes will call for placing them in a deep bowl, covering generously with boiling water and letting stand for about 15 to 20 minutes, then draining well.
To Griddle-Roast Onions and Garlic: Heat a heavy ungreased griddle or cast-iron skillet over medium-low heat until a drop of water sizzles on contact. Place the unpeeled onion and individual unpeeled garlic cloves on the griddle. Cook, turning frequently, until somewhat softened, 15 to 20 minutes for small onions, 20 to 25 minutes for medium-size onions and 8 minutes for garlic cloves. I usually cut large onions in half crosswise (leaving the skin on) and roast, turning frequently, for about 20 minutes. The cut side will char, but the black bits are easily scraped off. Remove the onions or garlic cloves from the griddle; when cool enough to handle, peel them over a bowl to catch the juices.
To Griddle-Roast Tomatoes: Heat the griddle or skillet over medium heat until a drop of water sizzles on contact. Place the tomatoes on the griddle stem side down. (Started this way, they lose less juice.)
Cook, turning frequently, until the skin is blackened and blistered all over, 10 to 15 minutes for small to medium tomatoes and 15 to 20 minutes for larger ones. Remove from the griddle to a bowl that will hold the juices. When they are cool enough to handle, peel off the charred skin. If some tiny blackened bits remain, they will just add to the flavor. Be sure to save all the delicious juices and add them to the dish.