To cook the pasta: Bring a saucepan of salted water to the boil. Add the pasta and cook as per package instructions.
To make the filling: Heat the oil in a frying pan over a medium-high heat. Add the bacon and fry until it's nice and crispy. Take the bacon out of the pan and set aside. Add the onion to the pan and cook for a couple of minutes. Then add the garlic, lemon juice, chicken stock, cream, mustard and seasoning to the pan. Scrape up any brown bits from the bottom of the frying pan. Bring to a simmer and then allow the sauce to simmer for 2-3 minutes to let it reduce and thicken.
Add the bacon back into the pan. Stir in the spinach and let it cook for a minute or two until the spinach is wilted. Taste and season.
To serve: Drain the pasta, reserving some of the cooking liquid. Return the pasta to the saucepan, add in the sauce and stir through three quarters of the cheese. Add a little of the reserved cooking liquid if the sauce is too thick. Divide the pasta between 4 pasta bowls and sprinkle over the reserved cheese and some fresh basil or parsley. Serve with crusty bread and a green salad.