• Preheat oven to 350°F

Make the bread crumb topping

• Melt 1 tablespoon of butter in a skillet over medium-low heat. When the butter is melted, add the bread crumbs. Stir well until the bread crumbs are moist. Remove the skillet from heat.

Make the cheese sauce

• Heat the milk in the microwave until warm.
• Melt the remaining 3 tablespoon of butter in a large sauce pan, over medium-low heat. Add the onions and cook gently for 2 minutes, stirring frequently.
• Add the flour to the pan and whisk until incorporated. Cook the flour for 1-2 minutes, stirring almost constantly.
• Gradually add the warm milk, whisking constantly until the milk is incorporated. 
• Stir in ¼ teaspoon salt, pepper, nutmeg and add the bay leaf.
• Raise the heat to medium and bring the sauce to a simmer (this should take about 5 minutes). As soon as you see a few bubbles, lower the heat to medium-low. Continue to stir the sauce, especially as it starts to thicken.
• Cook the sauce for an additional 3 minutes, stirring frequently.
• Remove the bay leaf from the sauce and discard.
  
• Add the cheddar and the Parmesan cheeses to the sauce and stir until melted and smooth.
• Keep the cheese sauce on low heat and give it a quick stir every so often so it doesn’t stick or burn on the bottom.

Make the broccoli

• Cook the broccoli in the microwave, following the package directions to the lowest recommended cooking time.
• Add the broccoli to a shallow baking dish and sprinkle with a pinch of salt.
• Pour the cheese sauce over the broccoli.
• Distribute the bread crumb topping evenly over the top.
• Bake the broccoli cheese casserole for 25-30 minutes, or until the topping is golden. Place the casserole dish on a baking sheet, just in case it bubbles over.
• Remove from the oven and let the broccoli cheese casserole cool for about 10 minutes before serving.