Combine melted butter and horseradish in a small bowl. Add panko crumbs and mix together with a fork until all crumbs are coated evenly.
Saute crumbs in a non-stick skillet for 5 minutes, until crisp, stirring often so as not to burn. Spread the crumbs on a baking sheet. Set aside.
Add 1/2 cup water and 1/2 tsp. kosher salt to a large saute pan. Add beans and bring to a boil. Cover and cook for 5 minutes. Remove lid and continue to cook until all water is almost evaporated.
Add 2 Tbsp. butter and 2 Tbsp. Dijon mustard Toss to coat. Season with the salt and pepper. Place beans on serving platter and top with horseradish crumbs. Serve immediately.