Preheat oven to 150 C.
Heat butter and oil in a small ovenproof pan over low heat until melted. Add the leeks and cook slowly, stirring occasionally to prevent sticking or burning. After 10 minutes add the spring onion, cook for a further 5 minutes until leeks and spring onion are soft. Remove from pan and leave to cool.
In the same pan (no need to wash) fry the rashers until crispy and golden, then remove and drain on kitchen paper.
Crack eggs into a bowl, season and whisk until completely blended. Whisk in the cheese/s and cream if using. Gently mix in the leeks, spring onion, diced bacon and chopped parsley.
Reheat the same small pan and lightly oil if required, removing any excess oil with kitchen paper before pouring in egg mixture. Cook on hob for about one minute, cover lightly with parchment and tin foil, then bake in oven until set and golden, about 30-45 minutes, removing the covering during the final 5 minutes (cooking time depends on depth of egg mixture and cooking container).
Serve with mixed baby leaves or simple green vegetables.