Season tuna with salt and pepper. Place in a saucepan with garlic and enough oil to just cover. Bring to a gentle simmer, and cook over low heat until the fish is opaque, about 10 minutes. Do not let it boil. Remove from heat, and add lemon thyme and bay leaves. Let stand until room temperature. Use immediately, or transfer to a bowl, cover with plastic wrap, and refrigerate overnight. Let stand at room temperature for 1 hour before using.