Beef and Horseradish Sauce Sandwich

INGREDIENTS

to 3 pounds fillet of beef, trimmed and tied

1 tbsp

unsalted butter, at room temperature

1 tsp

Dijon mustard

2 tsp

kosher salt

2 tsp

coarsely ground black pepper

loaf health or 7-grain bread

Mustard Horseradish Sauce, recipe follows

bunch arugula

Kosher salt

Freshly ground black pepper

Unsalted butter at room temperature

3/4 cup

good mayonnaise

1 1/2 tbsp

Dijon mustard

1 tbsp

whole-grain mustard

1/2 tbsp

prepared horseradish

2 tbsp

sour cream

Kosher salt