In Melnik we haven't found a restaurant that stands out of the crowd. There are numerous small restaurants scattered around the village, but the dishes they are preparing are nothing to rave about. The ingredients they use are nevertheless as fresh and good as elswhere at Bulgarian countryside, so you probably can't go wrong by picking any of the restaurants.
We ate a few times in the restaurant Mario. If you walk through Melnik in the direction away from the main road, Mario is on your left. There is a covered porch with a few tables in front of the restaurant. Among others we ate grilled sausages, grilled pork and fried potatoes with grated cheese. We also tried their house wine, which was OK, but we expected more from Melnik in terms of wine.
During one of the dinners we've been amusing ourselves watching local men occupying all other tables in the porch, watching a (supposedly important) football match on the TV. They got louder with every additional beer they had, emotionally commenting the action on the screen. It felt like for the villagers of sleepy Melnik football is the most exciting thing in the world.