Preheat oven to 350°F and grease a 9x13-inch baking dish.
Blanch broccoli florets in boiling salted water for 3 minutes, then drain well and pat dry.
In a large bowl, combine cream of mushroom soup, shredded cheddar cheese, mayonnaise, diced onion, beaten eggs, salt, and pepper, stirring until well mixed.
Fold in the broccoli florets and transfer the mixture to the prepared baking dish.
In a small bowl, mix crushed Ritz crackers with melted butter until evenly coated.
Sprinkle the buttery cracker mixture evenly over the top of the casserole.
Bake for 30-35 minutes until the top is golden brown and the casserole is bubbly around the edges.
Let stand for 5 minutes before serving.