Heat the oil in a pan over a medium heat and fry the onion for 8-10 minutes until softened. Add the garlic, the leftover vegetables and potatoes, and fry for 5 minutes, stirring occasionally.
Pour the stock into the pan and simmer for 10 minutes. Using a handheld blender, whizz the soup until smooth. Season with freshly ground black pepper.
Preheat the grill to medium. Top the bread with the Stilton and grill for 3-4 minutes. Garnish the soup with parsley and serve with the Stilton crostini.