In a large pot over medium-high heat, melt the butter with the olive oil. Add the diced onion and cook, stirring often, until softened, about 2 to 3 minutes. Stir in the minced garlic and cook until fragrant, roughly 1 minute.

Add the sliced cremini mushrooms and 2 teaspoons of the chopped thyme. Cook, stirring occasionally, until the mushrooms release their liquid and start to brown, about 5 minutes. Pour in the Marsala wine and let it cook down for 3 minutes, scraping up any browned bits from the bottom of the pot.

Sprinkle the flour over the mushroom mixture and stir well to coat. Cook for 2 minutes to remove the raw flour taste. Slowly pour in the chicken broth while stirring continuously, then bring the soup to a boil. Reduce the heat to low-medium, add the salt, pepper, and crumbled beef bouillon cubes, and stir to combine.

Cover the pot and let the soup simmer gently for 10 to 15 minutes, stirring occasionally, until it thickens to your liking.

Reduce the heat to low and stir in the heavy cream. Warm the soup gently without letting it boil. Taste and adjust the salt and pepper as needed.

Remove from heat and stir in the chopped parsley and remaining 2 teaspoons of thyme. Ladle into bowls and serve warm.