In a pot, melt the butter over medium-low heat. Add in the olive oil and chopped onion. Cook the onion over low heat for 3 minutes, stirring frequently. The onion will be soft and translucent. Add in the garlic and cook for another minute.
Add in the plain flour and stir to mix the onion, garlic and flour together. Cook for another minute over low heat. Stir in the chicken stock and add in the chopped Portobello mushrooms. Turn up the heat and bring the soup to a boil.
Once the soup is boiling, turn down the heat to low and cover the pot. Let it simmer for 15 minutes. The mushrooms would have all shrank in size. Add in the thyme leaves.
Scoop the mushrooms with a some soup (be careful it's hot!) into a blender and blend until the mushrooms are finely chopped. If you like to have bits and pieces to chew on, blend only most of the mushrooms.
Return the blended mixture back to the pot and bring the soup back to a boil. Turn the heat down to low and stir in the thickened cream. Season with salt and freshly ground black pepper.
Ladle the soup into serving bowls. Sprinkle some chopped parsley on top and serve with garlic bread immediately.