Cut bacon into small pieces and dice onion finely. Heat a large pan over medium heat and render the fat from the bacon. Once it is crispy, remove from the pan and drain on paper towels. Set aside.
Reduce heat to medium low and sauté onion in bacon grease without coloring until tender. In a large bowl, whisk egg with cream. Season with salt & pepper.
While onion cooks, cook spaghetti until al dente. Drain and add hot pasta to pan with onions.
Top with cream mixture, cooked bacon, parmesan cheese and butter and toss to completely coat pasta. {Might take a few minutes to get everything well coated.} Sauce should be very light. Garnish as desired with fried oregano, parmesan shaves & pancetta strips.