Preheat oven to 180°C. Lightly coat an 8-inch (2-L) baking dish with vegetable oil. Grate potato. Thinly slice onion diagonally. Cut tomato into rounds.
In a large bowl, whisk eggs with seasonings. Add potato, shallots and capsicum. Crumble in feta. Stir until mixed. Turn into baking dish and stir to evenly distribute ingredients. Top with tomato rounds.
Bake frittata, uncovered, in centre of oven until middle is set when dish is jiggled, 30 to 35 min. Take out of the oven and sprinkle with dill leaves.
Let the frittata cool for about 5 minutes, then slice and serve.