Melt 4 tablespoons butter and oil in a large heavy bottomed pot.
Add the carrots, celery and garlic and sweat for 5 minutes, just slightly browning the ingredients. Toss in the pearl onions and sweet potato.
Add the thyme, nutmeg and sage. Stir the vegetables to coat and then sprinkle with the flour. Mix to combine.
Add the vegetable stock and bring up to a boil.
Add in the swiss chard and the bay leaf.
Simmer for about 10 minutes.
Stir in the cream off the heat and adjust the seasoning.
Remove bay leaves and sage. (They are big and easy to find and fish out.)
Ladle out and serve.