Place the baby potatoes down into the bottom of a slow cooker.
In a skillet, heat the EVOO over medium-high heat. Season the beef chucks with salt and pepper and brown on both sides, then place it into the slow cooker pot along with the potatoes. In the same skillet, brown up the mushrooms; add them to the pot with the meat. Using same skillet again, sauté the onions, garlic and thyme until softened and fragrant. Remove to the pot with meat and mushrooms and top with the stock. Season with salt and pepper. Cover and cook undisturbed, for 4 hours high or 6-8 hours low.
In a small bowl, mix up the sour cream with the horseradish, chopped chive, salt and pepper; set aside.