In a large pot, heat oil or butter over medium heat. Add diced onion and cook until translucent, about 5 minutes.
Add mushrooms. Cook until mushrooms release their moisture and begin to brown, about 8–10 minutes.
Sprinkle flour and thyme over the mushrooms, stir well, and cook for another minute.
Pour in the milk while stirring with a whisk. Stir in beef stock powder.
Bring to a simmer, stirring consistently. It is ready once it has thickened
Once thickened, stir in the cream.
Use an immersion blender to puree the soup to your desired texture (smooth or a bit chunky).
Season with salt and pepper.