Prep Time: 45 minutes
Preheat the oven to 375 degrees. Bring a large pot of salted water to a boil over a high flame. Add the green beans and stir for 4 minutes. Prepare a large bowl with ice water and transfer the string beans using a slotted spoon. Drain well and pat dry.
In a large sauté pan over a medium flame, melt the butter. Add the flour and stir for 3 minutes, until slightly tanned. Add the milk and horseradish, along with a pinch of salt and pepper, and continue to stir until thickened, about 5 minutes. Remove from the heat and stir in the string beans and cheese, until thoroughly melted and combined. Pour into a casserole dish and place in the oven for 15-20 minutes, until bubbly and slightly browned. Top with chives.