Preheat the oven to 220°C, fan 200°C, gas 7. To make the batter, sift the flour and salt into a large bowl. Make a well in the centre. Beat together 2 whole eggs and one egg white (reserving the extra yolk for another recipe) and add to the bowl. Whisk in the eggs, gradually drawing in the flour and adding just enough milk to make a paste. When all the flour has been incorporated, whisk in the remaining milk until the batter is as thick as double cream. Leave to stand while you cook the sausages.
Place the sausages in a roasting tin and drizzle over the sunflower oil. Cook in the oven for 10 minutes, until the sausages are golden and the oil is very hot. Meanwhile, place the tomatoes on a baking tray. Drizzle with the olive oil and toss to coat. Carefully remove the sausages from the oven and pour the batter mix around them. Return to the oven, along with the tomatoes, and cook for 20-25 minutes, until the batter has risen and is golden brown.
Meanwhile, to make the gravy, heat the olive oil in a frying pan and fry the onion over a low-medium heat for 15 minutes, until very soft. Stir in the sugar and cook for another 5 minutes. Stir in the flour, then gradually add the stock, stirring continuously. Bring to the boil and simmer gently for 4 minutes, adding the mustard, thyme and Worcestershire sauce for the last minute. Season to taste with salt and freshly ground black pepper.
Meanwhile, cook the beans in a pan of boiling water for 3-4 minutes, until tender. Drain and serve with the toad in the hole, tomatoes and gravy.