In a large bowl, mix together risotto, Parmesan, cream, parsley and egg yolk. Slice cheese into small cubes. Scoop up about 3 tablespoons of the risotto mixture and, with your hands, pack it firmly around a 3-4 cubes of cheese forming a ball, 2-3 inches in diameter. Repeat until risotto is used up. In three separate bowls, place the flour, egg wash, and breadcrumbs. Roll Arancini in the flour, dip in the egg wash, and roll in breadcrumbs. Chill balls about 2 hours before frying. Fill a pot with 3 inches of vegetable oil. Heat oil over medium-high heat until temperature reaches 375 degrees. Deep-fry Arancini one at a time until browned, about 1-2 minutes. Using a slotted spoon, transfer to paper towels to drain. When all are fried, bake at 400 degrees for about 10 minutes (for cheese to melt inside.) Spoon tomato sauce on a plate and place Arancini on top.