4 bakery-fresh or store-bought, defrosted puff-pastry shells
1/4 cup Dijon mustard
1/2 to 3/4 teaspoon chipotle powder
1 teaspoon light-brown sugar
Juice from 1/2 lime
1 1/2 pounds top round sirloin steak (1/2 to 3/4 inch thick)
2 teaspoons extra-virgin olive oil, plus more for greens
1/4 teaspoon each: kosher salt and freshly ground black pepper, divided, plus more for greens
Mixed salad greens, for serving
If the pastry shells are unbaked, preheat oven to 425 F and bake according to package directions. Prebaked shells may be warmed in a 250-degree oven until ready to serve.
Whisk the mustard, chipotle powder, sugar and lime juice.
Cut the steak diagonally into 2-inch pieces, trimming off excess fat; pieces should be shaped almost like arrowheads. Drop them into mustard mixture and toss to coat evenly.
In a large, nonstick skillet over medium-high heat, heat the 2 teaspoons oil. Once oil shimmers, add half of steak. Season with half of the salt and pepper. Cook for 2 or 3 minutes, until steak pieces are seared in spots, then turn them over and cook for 1 or 2 minutes (medium-rare); be careful not to overcook meat, or it will be tough. Transfer to a plate. Add remaining steak pieces to skillet and repeat cooking and seasoning.
Reduce heat to medium-low; return first batch of steak to skillet just to warm through.
Lightly toss the greens with a drizzle of oil and a small pinch of salt and pepper. Place a pastry shell on each plate. Divide salad among shells; top each with steak.
Makes 4 servings.