Season sirloin with 1/2 tsp salt and 1/4 tsp freshly cracked black pepper. Heat oil in a large stainless skillet over medium-high. Cook steak 12 minutes, turning once, for medium-rare. Transfer steak to a cutting board to rest for 10 minutes. Meanwhile, reduce heat to medium. In the same skillet, add shallots and thyme; sauté 1 minute. Stir in blueberries and blackberries, stock, wine and vinegar. Season with salt and pepper. Bring to a simmer and cook 8 minutes, until berries burst and sauce is thickened. Stir in butter and season with salt and pepper. Slice steak and serve with sauce and a baby spinach and feta salad.