Place the potatoes in a saucepan, cover with water and cook over medium-high heat. Season with salt. Bring the potatoes to a boil, reduce to medium heat and cook until tender, about 12 minutes. Remove and drain. Place the potatoes back in the saucepan, over medium heat. Continue to cook for 1 minute, stirring constantly. Add the butter and cream. Using a hand masher, mash until the potatoes are slightly smooth. Season with salt and pepper. Add the garlic and corn. Mix well. Set aside, keeping warm. Season the tuna with salt. In a coffee grinder, add the peppercorns and coarsely grind. With a small knife, spread a tablespoon of mustard on one side. Pack the peppercorns into the steak. Turn the tuna over and spread another tablespoon of mustard. Pack the peppercorns into the tuna on the second side. In a large skillet, over medium heat, add the oil. When the oil is hot, add the tuna and sear for 2 to 3 minutes on each side, for medium rare. Place a mound of the potatoes in the center of each plate. Place the tuna on top of the potatoes. Garnish with parsley.