Preheat oven to 400 degrees F (200 degrees C). Grease a 9x13-inch baking dish.
In a bowl, mix together the mayonnaise, Dijon mustard, Italian dressing, and chopped onion until thoroughly combined; set aside.
Sprinkle the salmon with seafood seasoning on both sides, and place skin-side down into the prepared baking dish. Spread the mayonnaise mixture over the salmon steaks, and sprinkle with bread crumbs.
Bake in the preheated oven until the fish is almost opaque all the way through (centers can be slightly red), 15 to 20 minutes. Remove from oven, sprinkle with Cheddar cheese, and return to oven until the cheese is melted and bubbling, about 10 more minutes.