Bring 6 quarts water to boil and add 2 tablespoons salt.
Place onion and pancetta in a 12 inch to 14 inch saute pan and cook over medium heat until both onions and pancetta are translucent, about 8 to 10 minutes. Add cream and cook 1 minute. Separate eggs, being careful to keep yolks whole. Cook spaghetti according to package instructions and drain well. Put hot pasta into pan and toss over medium heat until coated. Add 1/4 cup pecorino and stir through. Remove from heat and vigorously stir in egg whites. Divide among 4 plates and top each with 1 yolk. Divide remaining grated cheese and black pepper atop each portion and serve.