Cook the pasta in a large pan of boiling generously salted water until al dente. Reserve a little pasta water before draining.
Meanwhile, grate the zucchini and use a clean tea towel to squeeze out the water.
Place a frying pan on a medium to high heat, melt the butter and add the mushrooms when the butter starts to bubble. Fry for a few minutes until they start to brown, stirring regularly to ensure they cook evenly.
Add the grated zucchini and garlic and fry for 1-2 minutes more.
Reduce the heat to medium and add the crème fraîche. Allow it to become hot for a minute or two and stir in the veg stock.
Add the black pepper and optional balsamic vinegar and simmer for a few more minutes.
Stir in the lemon juice and zest.
Toss the cooked pasta in the sauce, add a little pasta water if it needs extra moisture.
Serve immediately with chopped fresh parsley and more ground black pepper if desired.