Squeeze all excess water out of the tuna and set aside.
Halve the tomatoes, remove the seeds and watery centers, then dice the flesh into small pieces.
Finely julienne the basil leaves. In a small bowl, mix the basil with olive oil, lemon juice, minced garlic, salt, and pepper. Let sit for 10 minutes to allow the flavors to develop.
Slice the mozzarella ciliegine.
Place food rings (about 4 inches in diameter) on serving plates. For each portion, layer half of the diced tomatoes on the bottom, followed by tuna, half of the basil oil, sliced mozzarella, the remaining basil oil, and finish with the remaining tomatoes.
Carefully remove the rings and serve immediately.