If you're looking for the end-all, be-all of upper-crust cuisine, your search ends here. Our version of this classic features a tender steak topped with mushrooms, then wrapped in puff pastry and baked to flaky, golden perfection. We'll walk you through how to put together this meal that's fancy enough for heads of state, but simple enough to fit within your busy schedule.
Peel, trim, and cut carrot into ½" pieces at an angle. Stem and coarsely chop thyme. Place a medium oven-safe pan over medium-high heat. Add 1 tsp. olive oil and carrots to hot pan. Cook, stirring often, until lightly browned, 3-5 minutes. Place pan in oven and roast until tender and golden brown, 15-18 minutes, stirring once halfway through. Remove from oven and season with half the thyme (reserve remaining for sauce), ¼ tsp. salt, and a pinch of pepper. While carrots roast, sear steaks.
Sear the Steaks
Pat steaks dry, and season both sides with ½ tsp. salt and ¼ tsp. pepper. Place a medium non-stick pan over medium-high heat. Add 1 tsp. olive oil and steaks to hot pan. Sear until browned, 2-3 minutes on each side. Transfer to a plate. Steaks will finish cooking in a later step. Reserve pan; no need to wipe clean.
Cook the Mushrooms
Slice mushrooms thinly. Return pan used to cook steak over medium-high heat. Add 2 tsp. olive oil and mushrooms to hot pan. Cook, stirring occasionally, until golden brown, 4-6 minutes. Season with ¼ tsp. salt and a pinch of pepper. Remove from burner.
Prepare the Puff Pastry
Remove puff pastry square from refrigerator. Use the back of a knife to draw diagonal lines on puff pastry square, ½" apart. Draw another set of lines in opposite direction to make a crosshatched pattern. Halve puff pastry and refrigerate again. Place steaks on prepared baking sheet. Place ⅓ the cooked mushrooms (reserve remaining for sauce) on top of each steak.
Roast the Steaks
Place puff pastry over width of mushroom-covered portion of steaks. Stretch puff pastry slightly so it tucks under bottom of steaks. Place on top rack in oven and roast until puff pastry is golden brown and steaks reach a minimum internal temperature of 145 degrees, 8-12 minutes. Remove from oven and rest 2 minutes. While steaks roast, make sauce.
Finish the Dish
Add demi-glace and ¼ cup water to pan with remaining mushrooms. Bring to a boil over high heat and cook until reduced by half, 1-3 minutes. Remove from burner and swirl in butter and remaining thyme. Place carrots on plate and put steak Wellington in front of carrots. Spoon mushroom demi-glace in front of steak.