Step 1
Add bacon to a large saucepan and fry over low heat until almost crispy, about 15 minutes.
Step 2
While bacon is frying, cook pasta in a large pot of boiling heavily salted water (not a pinch of salt; handsful). Stop cooking pasta just before al dente. The pasta will finish cooking in the sauce. Scoop out 250 ml (1 cup) of pasta water just before the pasta is done and set aside.
Step 3
Add the olive oil and butter to the bacon. Stir in garlic, leek, lemon zest, and chili. Sauté over medium-low heat until aromatic and fragrant, about 5 to 7 minutes. Add the 250 ml of pasta water along with the cooked spaghetti. Stir in the lemon juice. Reduce heat to low and slowly stir in the beaten egg. Stir vigorously, making sure the egg does not scramble. Cook, stirring constantly, over low heat until sauce is thick and glossy, about 3 minutes. Season with salt and black pepper to taste. Serve with finely grated parmesan cheese.
