Cut the mushrooms into small cubes and finely chop the onion.
Heat the butter and olive oil in a pan.
Add the onion and sauté until translucent.
Add the mushrooms and cook until softened.
Set the mushrooms aside.
For the sauce, bring the cream, soured cream, thyme, 20g Jarlsberg, a little chopped parsley and salt to a gentle boil, then take off the heat straight away.
Pour the sauce into the pan with the mushrooms and simmer until it thickens and the mushrooms have soaked up the flavour.
Taste and adjust the salt.
Divide between your pots, top generously with the remaining 80g Jarlsberg, and bake at 220°C for 10 minutes, until golden and bubbling.
Mix the garlic, parsley and olive oil, and spread over the top just before serving.
