Preparing the mojo de ajo: Either chop the garlic with a sharp knife into 1/8-inch bits or drop the cloves through the feed tube of a food processor with the motor running and process until the pieces are roughly 1/8 inch in size. You should have about 1/2 cup chopped garlic. Scoop into a small (1-quart) saucepan, measure in the oil (you need it all for even cooking) and 1/2 teaspoon salt and set over medium-low heat. Stir occasionally as the mixture comes barely to a simmer (there should be just a hint of movement on the surface of the oil). Adjust the heat to the very lowest possible setting to keep the mixture at that very gentle simmer (bubbles will rise in the pot like sparkling mineral water) and cook, stirring occasionally, until the garlic is a soft, pale golden (the color of light brown sugar), about 30 minutes. The slower the cooking, the sweeter the garlic.
Add the lime juice to the pan and simmer until most of the juice has evaporated or been absorbed into the garlic, about 5 minutes. Stir in the chiles, then taste the mojo de ajo and add a little more salt if you think it needs it. Keep the pan over low heat,so the garlic will be warm when the shrimp are ready. Scoop the lime wedges into a serving bowl and set on the table.
The shrimp: Devein the shrimp if you wish: One by one, lay the shrimp on your work surface, make a shallow incision down the back and pull or scrape out the dark (usually) intestinal tract.
Set a large (12-inch) heavy skillet (preferably nonstick) over medium-high heat and spoon in 1 1/2 tablespoons of the oil (but not any garlic) from the mojo. Add half of the shrimp to the skillet, sprinkle generously with salt and stir gently and continuously until the shrimp are just cooked through, 3 to 4 minutes. Stir in half the cilantro or parsley, if you're using it. Scoop the shrimp onto a deep serving platter. Repeat with another 1 1/2 tablespoons of garlicky oil and the remaining shrimp.
When all of the shrimp are cooked, use a slotted spoon to scoop out the warm bits of garlic and chiles from the mojo pan, and scatter them over the shrimp. (You may have as much as 1/3 cup of the oil left over, for which you'll be grateful--it's wonderful for sauteing practically anything.) If you're a garlic lover, you're about to have the treat of your life, served with the lime wedges to add sparkle.
Copyright Mexico: One Plate at a Time (Scribner, 2000) by Rick Bayless