Preheat the oven to 180°C (gas mark 4) and grease a 23 cm x 33 cm baking dish.
Steam the broccoli florets for 5–7 minutes until just tender, or blanch in rapidly boiling water for 2–3 minutes; drain well.
In a large bowl, combine the condensed cream of mushroom soup, mayonnaise, 100 g of the shredded mature cheddar, finely chopped onion, garlic powder, black pepper and salt; stir until smooth.
Gently fold in the broccoli, then spread the mixture evenly in the prepared dish.
Toss the crushed crackers with the melted butter.
Sprinkle the remaining 50 g cheddar over the broccoli mixture, then evenly scatter the buttered cracker crumbs on top.
Bake for 25–30 minutes, until bubbling around the edges and the topping is golden-brown.
Leave to stand for about 5 minutes before serving.