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.