Preheat and Prepare the Broccoli: Set your oven to 180°C (350°F). Grease a 23x33cm baking dish with a little butter. Bring a large pot of salted water to a rolling boil. Add the broccoli florets and blanch for exactly 2 minutes. You’re looking for them to turn a bright, vivid green and become just tender when pierced with a knife, but they should still have a slight resistance. Drain immediately into a colander and rinse with cold water to stop the cooking process. Set aside.

Sweat the Aromatics: In a large frying pan, melt 1 tablespoon of butter over a medium heat. Add the finely chopped onion and cook for about 5 minutes, stirring occasionally, until it becomes translucent and soft, and you can smell its sweet, savoury aroma. Add the minced garlic and cook for a further 30 seconds until fragrant.

Build the Creamy Sauce: In a large mixing bowl, combine the tin of condensed mushroom soup, the sour cream, Dijon mustard, black pepper, and salt. Whisk everything together until it is completely smooth and a uniform pale beige in colour. Fold in the sautéed onion and garlic, then gently stir in 250g of the grated cheddar cheese until it is evenly distributed.

Combine and Assemble: Add the blanched and drained broccoli florets to the bowl with the sauce. Using a large rubber spatula, fold everything together gently but thoroughly, ensuring every single floret is coated in that luscious, cheesy sauce. The mixture should look thick and creamy, with bright green and white flecks throughout.

Transfer to the Dish: Spoon the broccoli mixture into the prepared baking dish and spread it into an even layer. Sprinkle the remaining 50g of grated cheddar cheese over the top, creating a first, melty blanket.

Make the Ritz Topping: In a small bowl, combine the finely crushed Ritz crackers with the 50g of melted butter. Stir with a fork until every crumb is moistened and the mixture resembles wet sand. Sprinkle this buttery crumb mixture evenly over the cheese layer.

Bake to Golden Perfection: Place the dish in the preheated oven, uncovered, and bake for 25 to 30 minutes. You'll know it's ready when you can see the sauce bubbling up around the edges of the dish, the whole casserole is steaming hot, and the Ritz topping has turned a deep, golden brown. The kitchen will smell gloriously of toasted butter and melted cheese.

The Crucial Rest: Remove the casserole from the oven and let it sit on the counter for at least 10 minutes before serving. This rest allows the sauce to settle and thicken slightly, meaning your servings will hold their shape on the plate rather than turning into a soupy puddle.