Blanch or steam broccoli florets for about 5 minutes.
In a stovetop pan melt the butter and sauté the onions for about 5 minutes. Dust in the flour and stir until moisture is absorbed.
Pour in the milk a little at a time and whisk or stir until a thick roux has formed.
Stir in the cream cheese, Dijon, salt and pepper until smooth.
Stir in the cheddar cheese until smooth.
Add the broccoli and stir again so all florets are coated. Transfer to a 9x13 casserole dish or a 3 quart casserole dish.
Crush the crackers to a medium crumb and add the melted butter. Toss to coat as best as possible.
Top the casserole and bake for 15 minutes until top is golden brown and cheese is bubbling. Remove from the oven and allow to rest for about 10 minutes to firm slightly for serving.
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Notes
To blanch broccoli, bring a pot of water to a boil. Place broccoli florets into the boiling water and boil for about 2 minutes. Remove the florets and immediately dunk them into a bowl of ice water. Allow the florets to sit in the ice water for another 3 minutes. Remove, pat dry, and continue with recipe instructions.