Preheat the oven to 180°C/160˚C fan/gas mark 4. Bring a large pan of water to the boil. Add the potatoes and cook for 15 minutes, until soft and tender. Drain well, return to the pan, then add the leftover roast potatoes and butter. Season well then roughly mash together.
Add the remaining veg, and combine everything together. Season well.
Lightly oil 12 muffin-tin cups and fill each with the bubble and squeak mixture. Lightly brush each with a little vegetable oil and bake for 40 minutes or until golden and crispy.
To serve (optional): while the bubble and squeaks are baking, cook the sausages and keep them warm. Poach the eggs in a large pan of boiling water. Wilt the spinach in a large pan then leave to drain in a sieve set over a bowl to get rid of excess water. Fry the tomatoes until golden and serve with the bubble and squeak, sausages and spinach.
TIP: These freeze well, so allow any leftover squeaks to cool then store in a tupperware box to freeze.