Add anchovies, olives, capers, parsley, and basil to the bowl of a food processor. Squeeze the juice of the lemon halves into processor, then add the lemon halves to the processor. Season with salt and pepper and pulse to combine. With the motor running, add the olive oil in a slow stream and continue blending until emulsified. Set aside.
In a large saucepan over medium heat, whisk polenta, 2 cups water and salt. When the mixture comes to a simmer, gradually whisk in the remaining water and bring to a boil. Reduce heat to simmer and cook for 30 minutes or until polenta is soft and creamy, about 25 minutes. Butter a large casserole dish and add polenta, smoothing it into a level surface. Cover with plastic wrap and chill for 6 hours.
Preheat a grill pan to medium. Cut polenta into 6 squares and then cut each square diagonally into 2 triangles. Brush polenta with olive oil and grill for 3 minutes per side or until heated through.
Set a grill pan over medium-high heat. Season the tuna steaks with salt and Essence and brush with olive oil. Place steaks on the grill, turning 45 degrees after 2 minutes. Repeat on the other side for an additional 2 minutes or until medium-rare. Remove from heat and set aside.
To assemble dish: Arrange polenta on plate. Stack tuna on top and spread tapenade on top of tuna. Garnish with parsley.