Preheat oven to 350 degrees F (175 degrees C).

Cook broccoli until tender-crisp. Steam or boil until slightly softened but still firm.

Cook onions in boiling salted water until tender. Drain well.

In a saucepan, melt half of the margarine over medium heat.

Whisk in flour, salt, and pepper until smooth.

Gradually add milk, stirring constantly, until the mixture thickens and becomes bubbly.

Reduce heat to low and blend in cream cheese until smooth and fully melted.

Place cooked broccoli and onions in a casserole dish.

Pour the cheese sauce mixture over the vegetables and mix gently to combine.

Top with shredded cheddar cheese.

In a separate bowl, melt the remaining margarine.

Toss the melted margarine with bread crumbs until evenly coated.

Sprinkle the bread crumb mixture over the casserole.

Bake in the preheated oven for 40-45 minutes, or until golden brown and bubbly.