According to a friend standards have really slipped at Dar Marjana and it's now an overpriced tourist trap trading on its past reputation so I would avoid that. How local are you willing to try - there are some small restaurants just off Djemaa el Fna which are open only at lunchtime that serve meshwi which is whole lamb roast vertically in and underground spit. Delicious.

I'll be interested in more suggestions as we're going back next March.