1 12-inch pain au levain or boule
1/2 cup good mayonnaise
2 tablespoons crème fraiche
2 tablespoons prepared horseradish, drained
3 teaspoons Dijon mustard
2 pounds roast beef, thinly sliced
3 cups fresh arugula
2 large heirloom tomatoes, thinly sliced
1 1/2 cups fried onions
Slice loaf horizontally and remove excess bread from the dome, reserving bread for crumbs at a later date.
Prepare the spread in a small mixing bowl by combining the mayonnaise, crème fraiche, horseradish and Dijon mustard. Season with salt and pepper to taste.
Build the sandwich by generously spreading both the top and bottom with the mayonnaise mixture. Layer the roast beef, tomatoes, arugula and fried onions.
Place the top on the sandwich and weight down with a heavy cast iron skillet for one to four hours.
Before packing for the picnic, tie with baker’s twine. Slice just before serving.