Porto Cervo's restaurants are some of the most expensive in Italy, and the quality varies more than the prices would suggest. The genuinely interesting cooking is usually outside the resort itself — in the hill villages behind the coast and in nearby Arzachena, where you can still find a proper Sardinian meal at sensible prices.
In the resort, the Pevero Golf Club restaurant is a reliable lunchtime option and open to non-members. The Pomodoro trattoria in the Sottopiazza is one of the more honest mid-range options in town, with good pasta and pizza. Spinnaker at Liscia di Vacca has been around since the 1960s, sits over the water, and is a fixture of the yachting set for a long lunch.
For the marquee experience, the Cala di Volpe hotel restaurant and the dining room at Pitrizza belong to the same Marriott Luxury Collection portfolio and serve at the level you would expect for the prices — book a long way in advance. For real value, drive twenty minutes inland: the agriturismi around San Pantaleo or Sant'Antonio di Gallura serve full Sardinian menus — porceddu (suckling pig), pane carasau, seadas — for a fraction of resort prices.