Place onion slices in 3- to 4-quart slow-cooker.
Spray 10- to 12-inch skillet with cooking spray; heat over medium-high heat. Lightly sprinkle beef with salt and pepper on both sides; sear on each side 1 minute. Place beef over onion. Sprinkle soup mix over beef. Add broth and soy sauce to slow cooker.
Cook on low heat setting 8 hours.
When ready to serve, bake biscuits as directed on can. Meanwhile, remove beef from slow cooker to cutting board or large plate. Using 2 forks pull beef apart to shred.
Cut each biscuit in half. Place beef and onion on bottom halves of biscuits. Serve sandwiches with cooking juices on the side for dipping.