Spray oven broiler rack with PAM. Preheat broiler.
Mix salt, thyme, pepper, and garlic powder in cup. Rub seasoning mixture all over steak. Transfer steak to broiler rack. Broil steak 5" from heat until lightly browned, about 5 minutes per side.
Scatter onion in bottom of 5- or 6-quart slow cooker Top with steak. Whisk broth, wine, and tomato paste in medium bowl until smooth. Pour broth mixture over steak. Cover and cook until steak and onion are fork-tender, 4 - 6 hours on high, or 6 - 8 hours on low.
About 35 mins. before cooking time is complete, heat olive oil in a large non-stick skillet over medium-high heat. Add mushrooms and cook, stirring frequently, until lightly browned, about 8 minutes. Sprinkle with flour and cook, stirring constantly, about 1 minute. Stir mushroom mixture into slow cooker. Cover and cook on high, until mixture simmers and thickens, about 25 minutes.
Transfer steak to cutting board. Cut steak across the grain into 12 slices (three slices per serving). Serve with broth over top.
Weight Watchers calculates this as 7 Points.
If you want to add a 1/2 cup of wild or brown rice and a side of steamed broccoli or broccoli-rabe to each serving, add 2 extra Points per serving.