Step 1 Line a rectangular metal baking pan (20 x 30cm wide, 5-6cm deep) with a sheet of baking paper fit square in the base of pan and up the sides, with corners folded (but not cut). Coat baking paper with oil spray. Preheat oven to 220°C (or 210°C fan bake) with the rack positioned in the middle.
Step 2 Cut potatoes in half lengthwise then in quarters crosswise. Cook in about 2cm of lightly salted water in a medium-sized pot, covered, for about 20 minutes or until potatoes are almost tender. Add peas and cook for 5 more minutes. Drain vegetables and spread them out on a shallow tray or dish to cool.
Step 3 When potatoes are cool enough to handle, cut the quarters in small chunks. Mix together onions, tomatoes, peas and potatoes then spread half the vegetable mixture over the base of the pan. Sprinkle over half the cheese then top with the remaining vegetable mixture.
Step 4 In a bowl beat together eggs, milk, and salt and pepper to taste. Pour evenly over the vegetable and cheese layers and sprinkle the remaining cheese over the top.
Step 5 Bake for 40–45 minutes until the egg mixture is set and the cheese topping has browned. Leave to cool for at least 1/2 hour before cutting with a serrated knife into about 12 slices, 3 portions crosswise and 4 lengthwise. Eat immediately or cover and refrigerate for up to 2 days until required.