@danchaq no problem. I too have played about a bit with it. I use slightly more sugar than salt now, maybe 60/40 with a generous amount of crushed or ground mixed peppercorns (maybe 3 tablespoons). I would use 100g of sugar and 75g of coarse salt for a 3lb fish. I use dill though maybe more sparingly than some. Juniper might be quite strong? The lemon zest sounds interesting. I have used it with hot smoked salmon before to rub into the surface, but not with gravadlax.
The other thing I do is make it with two fillets rather than one, so they can be ‘sandwiched’ together. This helps get the cure right into the fish. As I said above, I use a largeish rainbow trout these days, maybe 3lb or so. I have also done it with a grilse (small salmon) of about 4lb but for bigger fish, ideally 7-8lb, I tend to get them cold smoked (haven’t tried that myself yet, though I do my own hot smoking).
For gravadlax I also freeze the fish first then thaw. This kills any possible parasites. Quite important as gravadlax process itself won’t necessarily do that. It’s an outside chance, but worth doing.
I have also been known to add a splash of vodka to the gravadlax cure. The curing timespan is also important. For smaller fish I have been known to do it for as little as 36-48 hours, mainly due to forgetting to start in time! This gives a pretty mild cure and it won’t keep for too long. More generally, I do it for 3-4 days for a fish of 3lb. If it was bigger, I would do it for maybe 5-6 days. That length of cure time will keep for at least a week, probably more like 10 days once it’s ready.
Important to turn it over in the cure mix (which will go liquid) to ensure that it all gets covered. You can wrap it in clingfilm to do that. Best to use a dish that the fish fits in fairly well, a large plastic box, or a wooden half wine case works well (though it will leak a bit). Keep it somewhere cool - in winter I use the garage if the fridge is full. Rinse and wipe off the cure mix when finished then dry. You can brush some cure mix back on the flesh side and coat with finely chopped dill/crushed pepper to give it a nice finished look.