Most travel blogs about food in Sri Lanka tell you to try "rice and curry" and "hoppers" and leave it there. That's like telling someone visiting Italy to try pasta. Here's what to actually order, where to find it, and what to avoid paying tourist prices for.
Start every morning with egg hoppers. A hopper is a bowl-shaped rice flour pancake, crispy at the edges, soft in the middle, with an egg cracked into it while it cooks. You eat it with pol sambol (fresh coconut, chilli, and lime) and a thin dhal. A plate of two hoppers costs around 200 to 350 rupees at a local spot. At a beachfront cafe in Mirissa, the same thing costs 600 to 900 rupees and isn't as good. Find the local hopper stalls. They usually open from 6am to 10am and again from 6pm to 9pm. If the stall is empty at 7am, that's a bad sign. If there's a queue, get in it.
For lunch, eat rice and curry packets. Not sit-down rice and curry — the wrapped kind. Banana leaf or plastic bag, bought from a small shop or roadside stall between 11am and 1pm. You'll get rice, three or four small curries (usually dhal, a vegetable, a fish or chicken dish, and a raw salad), and a pappadam. The whole thing costs 150 to 250 rupees. It's the best meal of the day and the most honest representation of how people actually eat here.
Kottu is the right dinner order after a long beach day. It's roti chopped and stir-fried on a hot iron plate with egg, vegetables, and meat or fish, cooked loud and fast in front of you. Kottu is everywhere. The best version isn't at the nicest-looking restaurant. It's at the place where the sound of the blades hitting the iron is loudest at 8pm. Order chicken kottu or fish kottu. Cheese kottu is a tourist menu item.
For seafood, go one street back from the beach. The restaurants with the best views charge the most and buy the cheapest fish. The best fish curry on the south coast comes from small family restaurants away from the waterfront. Ask your guesthouse owner where they eat. That question alone will lead you somewhere better than anything on TripAdvisor.
Curd and treacle is the dessert you'll remember. Buffalo curd with kithul treacle — a dark, smoky palm syrup — served in a clay pot. Available at most local restaurants and some roadside stalls. It costs almost nothing and is one of the genuinely distinctive food experiences on the island.
The tourist trap pattern is consistent. Any restaurant with laminated photos on the menu, a tout at the door, or a sunset view from the terrace is charging at least double. The food is usually fine. You're paying for the location. Know that going in and decide accordingly.