Swiss chard is much like spinach but with larger and sturdier leaves. It provides a neat package for the tomato and melted mozzarella inside.
Remove 20 of the largest Swiss-chard leaves from the heads without tearing and set aside. Reserve remaining chard for another use.
Bring a large pot of lightly salted water to a boil. Have a bowl of ice water ready nearby. Plunge one or two leaves into boiling water for about 10 seconds, just until wilted. Place in ice water until cool, then drain on paper towels. Repeat with remaining leaves.
Cut tomatoes into 1/4-inch slices. Cut cheese into 1/4-inch slices and trim to approximate size of tomato slices.
Start a medium-size charcoal fire; when coals are hot, place grill as close to them as possible.
To assemble Swiss-chard packets, first place a Swiss-chard leaf facedown on a work surface. Cut out thickest part of stalk to about halfway up the leaf. Place a tomato slice in center of Swiss-chard leaf, top with slice of cheese, sprinkle with salt and pepper, and drizzle with olive oil. Fold bottom part of leaf over contents, then fold in sides; fold down top to enclose contents completely. Brush both sides lightly with olive oil, and place on grill. Grill for about 1 to 2 minutes on each side, until lightly charred and heated through. Serve immediately.