When you are in a restaurant, click on the bar that shows Last Month's Profit, then click on the pie chart button. It will show how much you sold of each dish and what profit you made off that dish.
Increase the price of the popular dishes. Keep increasing the price until customers specifically complain that that particular dish is expensive.
You can earn around $17-20 profit on some Main dishes.
Cinnamon Streusel Buns are extremely popular in French restaurants. I charge $20 for that dish and it's still the most popular breakfast dish (open your restaurant at 6AM).
Eggplant, Zucchini ... Torte is a good dish for profit and popular.