Trim excess fat from the sirloin steaks and season them lightly with black pepper. Avoid adding salt at this stage because the condensed soup, onion soup mix, and gravy mix already contain substantial sodium.

Place the sliced onion and optional mushrooms in the bottom of the slow cooker. Arrange the seasoned steaks over the vegetables in a mostly even layer.

Whisk together the cream of mushroom soup, beef broth, dry onion soup mix, and brown gravy mix until smooth. Pour the mixture over the steaks, making sure the sauce is distributed across the top.

Cover and cook on LOW for 6 to 8 hours, or on HIGH for about 3 to 4 hours. LOW is preferred because sirloin can become firm or dry if it is overcooked on HIGH.

Check that the gravy is hot and thickened and that the steak has reached at least 145°F internally, followed by a 3-minute rest for food safety. For the softer braised texture expected here, the steaks are generally cooked considerably longer.

Taste the gravy before serving and add salt only if it truly needs it. Spoon the steak and plenty of gravy over mashed potatoes, rice, or egg noodles.