Preheat the oven to 180°C, fan 160°C, gas 4. Heat the oil in a 20cm ovenproof frying pan and cook the onion for 5 minutes, until softened. Add the mushrooms and continue to cook for a further 5 minutes. Stir in the garlic and peas and cook for 1-2 minutes before transferring to a large bowl.
Beat the eggs together with the crème fraîche, then stir into the onion and mushroom mix along with the potatoes. Season with freshly ground black pepper and pour back into the frying pan. Scatter with cheddar and cook over a medium heat for 5 minutes. Transfer to the oven and continue to cook for 15-20 minutes, until set. Serve in slices with the salad and tomatoes.