Beef Wellington Sandwich

Ingredients

For 4 servings

• •4 large croissants, homemade or store-bought, split
• •1 /4 cup (60 ml) mayonnaise
• •1 /4 cup (60 ml) Dijon mustard
• •2 c. à soupe (30 g) prepared horseradish
• •1 c. à soupe unsalted butter (1 ounce; 14 g)
• •1 small shallot (1 1/2 ounces; 45 g), thinly sliced crosswise
• •16 ounces (454 g) cremini or button mushrooms, stemmed and cut into 1/4-inch slices
• •1 c. à café minced fresh thyme leaves
• •1 /2 teaspoon Diamond Crystal kosher salt, plus more to taste; for table salt, use half as much by volume
• •1 /4 teaspoon freshly ground black pepper, plus more to taste
• •12 ounces (340 g) thinly sliced rare roast beef, homemade or store-bought (see notes)
• •4 ounces (113 g) thinly sliced prosciutto
• •1 ounce baby arugula (28 g; 2 cups)

Method

1. 1.Adjust oven rack to middle position and preheat to 375ºF (190°C). On a 13-by-18-inch rimmed baking sheet, place croissants cut-side up and toast until edges are lightly browned and slightly crisp, 6 to 8 minutes. Set aside.
2. 2.In a small bowl, whisk together mayonnaise, mustard, and horseradish. Season with salt and pepper to taste. Set aside.
3. 3.In a medium skillet over medium high heat, melt butter. Add shallots, mushrooms, thyme, salt, and pepper. Cook, stirring occasionally, until mushrooms are well browned, 6 to 8 minutes.
4. 4.To Assemble: Using a butter knife or offset spatula, spread 1 tablespoon mayonnaise sauce evenly over 1 side of each croissant half. Top each bottom half with 3 ounces roast beef (about 2 slices), 1 ounce prosciutto (about 2 slices), and 1/4 of the arugula. Divide mushrooms equally (about 1/3 cup each) between the top halves and carefully flip top half of croissant onto each bottom half to close the sandwich, pressing gently to secure.