Vegetables Wellington

Preheat the oven to 350 degrees F (175 degrees C). Drizzle sweet potatoes with 1 tablespoon melted butter and sprinkle with 1/4 teaspoon each salt and pepper. Wrap each potato in foil and put on a 10x15-inch rimmed baking sheet. 

Bake in the preheated oven until tender, about 1 hour. 

Let sweet potatoes cool completely, about 1 hour. (Baked sweet potatoes can be chilled up to 5 days. Let chilled potatoes stand at room temperature 30 minutes before assembling in Step 5.)

Meanwhile, add 1 1/2 cups broth, rice, garlic, and 1/4 teaspoon salt to a medium saucepan. Bring to a boil over medium high heat. Reduce heat to low. Simmer, covered, until rice is tender, 40 to 45 minutes; drain any excess water. 

Meanwhile, melt 1/4 cup butter in a very large skillet over medium heat. Add mushrooms and onion. Cook, stirring occasionally, until liquid has evaporated, mushrooms are starting to brown, and onion is tender, 10 to 12 minutes. Add sherry; cook until evaporated, about 2 minutes. Stir in tarragon and remaining 1/2 teaspoon each salt and pepper. Stir in rice mixture and goat cheese until combined. Spread in an even layer on baking sheet and let cool completely. (If filling is hot, pastry will tear and not bake properly.)

Preheat the oven to 350 degrees F (175 degrees C). Cut each puff pastry sheet into quarters to make 8 pieces. Roll 6 pieces to 6 1/2 inch squares on a lightly floured surface. Evenly pile filling onto center of each square (about 1/3 cup per square). Top each with a sweet potato.

Line a clean baking sheet with foil. Beat together egg and water in a small bowl. Brush edges of topped pastry squares with egg wash. Bring up corners of pastry and gently stretch over potatoes; pinch edges to seal. Arrange bundles, seam sides down, on prepared baking sheet. Cut decorative shapes from remaining 2 pastry pieces. Arrange pastry shapes on bundles and brush with remaining egg wash to adhere. 

Bake in the preheated oven until golden brown and heated through, 20 to 25 minutes. (An instant read thermometer inserted into centers is the easiest way to test doneness. Thermometer should register at least 165 degrees F (74 degrees C). Let stand 10 minutes. Serve with mushroom gravy.