In a large pan, heat the olive oil over medium heat. Add the onion and sauté until translucent, about 2-3 minutes.
Add the garlic and thyme, and cook for another minute until fragrant.
Increase the heat to medium-high and add the sliced mushrooms to the pan. Sauté the mushrooms until golden-brown and have released their moisture, about 5-7 minutes.
Pour in the heavy cream and stir to combine with the mushrooms. Simmer gently for 3-4 minutes or until the cream has thickened slightly—season with salt and pepper to taste.
While the mushroom mixture simmers, toast the sourdough bread until it’s golden and crispy.
To serve, spoon the creamy mushroom mixture over the toasted sourdough slices. Garnish with chopped parsley and, if desired, a drizzle of truffle oil for an extra touch of luxury.