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1
Peel shallots, cut in half lengthwise, then cut crosswise into 1/2-inch-thick slices.
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2
Melt butter with 1 1/2 tablespoons oil in a large, heavy frying pan over medium heat.
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3
Add shallots; reduce the heat to medium-low.
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4
Cook, stirring frequently, until shallots are nicely browned, translucent and tender, about 12 minutes.
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5
Remove from heat.
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6
Sprinkle one side of each steak with salt and pepper to taste.
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7
Place a heavy medium frying pan over high heat for 3 to 4 minutes, until pan is very hot.
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8
Add the remaining 2 tablespoons oil, then sear the steaks for 1 minute per side.
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9
Transfer steaks to a dinner plate; let cool for 5 minutes.
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10
Top each cooled steak with a generous teaspoon of mustard, one-quarter of the shallots and 1 tablespoon cheese.
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11
On a floured work surface, roll puff pastry into a 12-inch square.
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12
Cut pastry into four 6-inch squares.
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13
Brush a 1/2-inch border around each square with beaten egg.
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14
Refrigerate the remaining egg.
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15
Place a pastry square (brushed-side down) over a steak; carefully lift the meat and press the pastry around its sides, then seal tightly on the bottom.
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16
Place on a (foil lined) lightly oiled baking sheet.
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17
Repeat with the remaining sheets.
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18
Refrigerate the Mini Wellingtons for 45 to 60 minutes.
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19
Preheat the oven to 425 degrees.
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20
Brush the tops and sides of the pastry enclosing each steak with some of the reserved beaten egg.
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21
Sprinkle the tops with a pinch of fleur de sel.
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22
Roast on the middle oven rack until the pastry is golden brown and a thermometer inserted through the pastry to the center of the meat registers 135 degrees to 140 degrees for medium-rare, 15 to 18 minutes.
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23
Transfer the Mini Wellingtons to a rack and let rest for 5 minutes before serving.
