Bring a large pot of well-salted water to a boil. Add fettuccine and cook according to package directions (usually 10–12 minutes). Add broccoli florets during the last 3–4 minutes of cooking. Before draining, reserve 1 cup of the starchy pasta water.
In the same pot (after draining), melt butter over medium heat. Add white wine and let it bubble and reduce for about 1 minute. Stir in the heavy cream.
Add the drained pasta and broccoli back to the pot. Add the lemon zest and toss everything together, adding reserved pasta water a splash at a time until the sauce is glossy and coats the noodles. Stir in the parmesan. Remove from heat, add the lemon juice, and season to taste with salt and black pepper.
The key is the pasta water — it helps emulsify the butter and cream into a silky sauce. Add the lemon juice at the very end to keep it bright and prevent the cream from looking broken. If the sauce looks greasy or separated, add more pasta water and toss vigorously.