Pour vinegar over fish and let rest for 10 minutes (you can also try the "soaking the fish in milk" method, though I do not have personal experience with this).
Preheat oven to 400 degrees. In a 9x13 dish, use two sheets of aluminum foil to make two bowls, one for each fish.
Place each fish in its aluminum foil bowl and stuff the insides with lemon slices, onion slices, olive oil, salt, dill, and vinegar. Also coat outside of fish with olive oil and vinegar
Place in oven and bake for about 45 minutes, until the fish is flaky.
When the fish has about five minutes left, begin making the lemon butter sauce. Melt the butter in a saucepan on the stove. Add in the rest of the ingredients and stir to combine. Pour over fish meat when the fish comes out of the oven or serve on the side.