Return your pan to the heat and add your butter. Once the foaming has subsided, add your flour.
Gradually add your milk, whisking frequently, until everything is incorporated.
Heat your 1 tbsp of olive oil over a medium heat and add your bacon. Fry until crispy then drain and set aside
Add your chicken stock. Bring to a boil then reduce to a simmer and cook for 15 minutes or so, or until thick and glossy.
Add your peas and stir to heat through.
Add salt, pepper, nutmeg, pasta, cooked bacon and stir through to combine.
Add all of your cheeses. Stir them through your pasta and remove from the heat.