Add the bacon to a large deep pan over medium heat and gently fry until crisp with the fat rendered down.
As the bacon begins to finish, add the pasta to salted boiling water and cook for a couple of minutes less than you usually would. It's essential the pasta is fairly al dente as it will continue cooking in the sauce. Drain the pasta, saving around a cup of the leftover pasta water.
Once the bacon is crisp, remove it and place to one side, leaving the excess fat behind. Melt the butter then add the grated courgette. Increase the heat slightly to medium-high then stir to coat the courgette in the butter/bacon fat. Add 1/2 tsp salt, pepper and lemon zest and fry for a few minutes until it wilts and begins leaking out water (should reduce by around half).
Stir in the stock, cream, Boursin, parmesan and 2 tsp lemon juice and bring to a simmer. Reduce the heat slightly and simmer until the liquid begins to thicken (3-5 mins).
Add the drained pasta and toss to coat in the sauce. It'll likely still be pretty watery at this point, but the sauce will gradually begin to reduce and thicken. Just continue gently simmering and tossing until the sauce turns into a creamy, glossy consistency and the pasta finishes cooking through. It will thicken so have faith! If you over thicken or need more liquid to finish cooking the pasta through just toss in a splash more pasta water as needed.
Stir through the bacon then check for seasoning and adjust if needed with salt and pepper. You can also add in more lemon juice if you feel it needs it.
Serve with extra parmesan if desired then tuck in and enjoy!