Step 1

Preheat the oven to 400 degrees F. Grease an 8-inch square baking dish or gratin dish.

Step 2

Bring 1 inch water to a boil over medium-high heat in a large saucepan with a steamer basket and lid. Add the broccoli to the basket, then cover and steam until the broccoli is bright green and tender, but still crisp, 4 to 6 minutes. Remove the broccoli to a plate and set aside.

Step 3

Heat the oil in a large saucepan or skillet over medium-high heat. Add the mushrooms and cook until they have released their moisture and started to brown, 4 to 5 minutes. Add the salt, pepper, onion powder and butter. Stir until the butter is melted. Sprinkle the flour over the mixture and stir until a paste forms. Cook until the paste is golden brown and has a nutty aroma, 3 to 4 more minutes. Slowly pour the milk into the mixture a little at a time, stirring as you pour, until the milk is all combined, then bring to a simmer and cook until the mixture thickens to coat the back of a spoon, another 3 to 4 minutes. Remove from the heat and add the broccoli, tossing to coat. Transfer the mixture to the baking dish.

Step 4

Put the butter crackers and cheese in a food processor and grind into a fine crumb mixture. Sprinkle the crumbs over the top of the broccoli and bake until bubbling and the topping is golden brown, 20 to 25 minutes. Let cool for a few minutes before serving.