Rinse the peppers and pat dry, and then place the whole peppers onto the grill, charring the skin completely on all sides until black on the outside and soft on the inside, 5 to 8 minutes per side. Remove and discard the charred skin, seeds and stems. Cut the peppers into thin slices about 1/4-inch thick.
Place the slices in a container with a lid. Drizzle with the vegetable oil. Add the salt, black pepper and garlic and mix well. Cover and refrigerate overnight.