Sprinkle the Cornish hens with 1 teaspoon salt on each side.
Combine the olive oil, lemon zest, lemon juice, garlic, rosemary, and 1 teaspoon pepper in a ceramic or glass dish just large enough to hold the flattened hens. Add the hens, and turn to coat. Cover the dish with plastic wrap and refrigerate for at least 4 hours or overnight, turning once or twice.
When ready to grill, prepare a hot charcoal fire on one side of a grill (or turn a gas grill on low heat). Spread 1/4 of the coals across the other side of the grill. Place the hens on the cooler side skin-side up and weigh them down with a gratin dish with a foil-wrapped brick inside.
Cook for 12 minutes, until the underside is golden brown. Turn the hens skin-side down, weigh them down again, and cook for another 12 minutes until the skin is golden brown and the hens are cooked through.
Place the lemon halves on the cool side of the grill, cut-side down, for the last 10 minutes of cooking. Remove the Cornish hens and lemons to a plate or cutting board, cover with aluminum foil, and allow to rest for 5 minutes. Cut the hens in half, sprinkle with fleur de sel, and serve with the grilled lemon halves.