In a small bowl, soak dried mushrooms in hot water.
Heat butter and 2 tablespoons olive oil in a skillet over medium heat. Stir in the onion and garlic; cook until the onion is tender and transparent. Stir in beef bouillon cube, and pour in mushrooms with water. Season with salt and pepper. Mix in cream; simmer gently for 5 minutes. Set aside.
Place whole potatoes in a pan with water to cover; bring to a boil over high heat, and cook about 5 minutes. Drain water. When potatoes are cool enough to touch, cut them in half. Heat 2 tablespoons olive oil in a skillet over medium heat, and fry potatoes until golden. Season with salt and thyme. Set aside.
Preheat oven to 400 degrees F (200 degrees C).
Season beef medallions with salt and pepper. Heat 2 tablespoons olive oil in a wok or large skillet over high heat. Place medallions in hot oil, and brown on both sides. Remove from heat, and place medallions in the center of a roasting dish.
Cover medallions with porcini sauce. Arrange potatoes around medallions. Cover dish with aluminum foil. Roast for 15 minutes.
