Steam green beans until tender-crisp, about 5 minutes.
Transfer to a bowl of ice water to cool.
Drain the beans, place on a clean kitchen towel and pat dry.
Preheat oven to 400F.
Coat a 9-by-13-inch baking dish with cooking spray.
Melt butter in the pot over medium heat.
Add flour; cook, whisking constantly, until starting to color, 2 to 3 minutes.
Slowly add milk, whisking constantly.
Cook, stirring often, until thick enough to coat the back of a spoon, about 6 minutes.
Remove from heat and, a handful at a time, whisk in fontina and Parmesan until smooth.
Stir in salt, mustard and pepper, then the green beans.
Scrape into the prepared baking dish.
Sprinkle with onions and bake until bubbling, 20 to 25 minutes.
Tips
To make ahead:Prepare though Step 3 and refrigerate for up to 2 days.
Let stand at room temperature while the oven preheats.
- Daily Values (DVs) are the recommended amounts of nutrients to consume each day.
(-) Information is not currently available for this nutrient.
