Put a large pot of salted water on the stove; bring to a boil. Meanwhile, heat up a large skillet on the stove and add the olive oil. Begin to brown the Italian sausage over medium heat.
When the water is boiling, put the pasta in the pot and cook for 8 to 10 minutes, until al dente.
While the pasta is cooking, finish browning the Italian sausage. Remove the sausage from the pan, leaving the fat in the skillet. Drain on a paper towel lined plate. In a small bowl, mix the beaten eggs with the parmesan cheese, stirring well to prevent lumps.
Turn the heat down in the skillet to medium low heat, toss the garlic into the hot pan and saute for less than one minute, to soften. Add the butter.
Add the hot al dente pasta to the skillet with the garlic--I use tongs to move the pasta directly from the pasta pot into the skillet. Using tongs, toss the pasta around for about 2 minutes to coat the strands with the fat in the pan. Add the splash of cream and the lemon zest. Remove from heat and pour in the egg/cheese mixture along with some of the reserved pasta water. Stir very quickly until the eggs thicken. At this point, add the cooked sausage back into the pan. Thin out the sauce with more of the reserved pasta water if necessary to desired consistency.
Season the pasta with the black pepper, taste to see if it needs salt. To serve, place the carbonara into serving bowls and garnish with the parsley and more freshly grated cheese. Mangia!