Spray a Dutch oven with cooking spray and preheat over medium heat.Brown meat, in batches.
Place meat in a 3 1/2-quart or larger slow cooker. Combine soup, onion, tomato paste, and lemon pepper in a bowl and stir to mix. Pour over the meat. Add mushrooms; cover and cook on LOW for 8 to 10 hours (or on HIGH for 4 to 5 hours).
About 30 minutes before serving, add frozen vegetables and buttermilk. Cover and cook on high for about 30 minutes. Serve over hot noodles.