To char the chiles: Over a gas flame, char the chiles on all sides until blackened, placing them into a sealable plastic bag to sweat for about 10 minutes. Remove the chiles from the bag and, using paper towels, wipe off the charred skin from the chiles. With a paring knife, slice the chiles down the center just to reveal the inside, being careful not to fully cut them in half. With a spoon, scoop out the seeds and the vein, keeping the stem and the rest of the chile intact. Place the chiles in a 9-by-13-inch glass baking dish and set aside for stuffing.
To make the tuna filling: Combine the tuna, peas, corn and mayonnaise in a small bowl. Season with salt and pepper and set aside.
To make the pickling liquid: Cook the carrots in boiling salted water until just crisp-tender, about 3 minutes. Drain and set aside to cool.
Heat the oil in a large saucepan over medium heat. When the oil is hot, add the garlic and onions and cook until the onions are slightly cooked, but still crispy, 5 minutes. Add 2/3 cup water and the vinegar and bring to a simmer (see Cook's Note, below). Add the carrots, oregano and bay leaves. Simmer until the vegetables are crisp- tender and almost all of the vinegar has evaporated, 8 minutes. Let cool slightly.
To stuff the peppers: Stuff the peppers with the tuna mixture. Place back in the baking dish; they should fit snuggly. Pour the warm pickled mixture over the stuffed chiles and allow to cool to room temperature. Cover and chill for 2 hours or overnight. Serve cold.
For safety, before adding the water and vinegar to the hot oil, sprinkle just a few drops of the liquid into the oil. If the oil splatters, then it needs to cool before adding the full recipe amount. Continue to test the oil with mere sprinkles of vinegar and water until no splattering occurs. At this time, it will be safe to add the full amount of vinegar and water.