Cook the new potatoes in a pan of boiling water until tender. Drain, cool slightly, then slice into thick rounds.
Heat 1 tbsp of the oil in a medium ovenproof frying pan over a medium heat. Add the onion and gently cook for 3-4 minutes, until soft.
In a large bowl, lightly beat the eggs, then add the cooked onion, potato rounds, tomatoes and cheese and stir to combine.
Preheat the grill to high. Add the remaining oil to the pan and pour in the egg mixture. Cook over a low heat for 8 minutes until the bottom begins to set.
Put the pan under the grill and cook for 8 minutes or until the centre is cooked through and firm. Let it cool in the pan, then cut into wedges.