Squeeze the lemon pulp and juice and scrape all the cooking juices from the baking dish into a strainer supported over a bowl. Force it through and add garlic. Whisk in rosemary and extra-virgin olive oil. Keeps up to 1 week refrigerated in a tightly sealed container.
Soak rosemary or wooden skewers in water to cover 1 to 2 hours.
Preheat grill or broiler. Skewer 2 tuna pieces lengthwise on each rosemary sprig and brush with marinade. Season with salt and pepper and grill, turning once or twice, to medium rare, 8 to 10 minutes, depending on heat. Serve immediately.