01 -
In a small bowl, combine Dijon mustard, sour cream, mayonnaise, minced garlic, and a pinch of salt. Whisk until smooth, adjust seasoning as needed, cover, and refrigerate until serving.
02 -
Preheat oven to 200°C. Line two sheet pans with parchment paper and set aside.
03 -
In a food processor, pulse mushrooms, shallots, and two garlic cloves until finely chopped. Scrape down the bowl as needed.
04 -
Melt 15 ml unsalted butter in a large pan over medium heat. Add mushroom mixture and sauté until very soft and liquid has evaporated to form a paste, about 8-10 minutes. Stir in 5 ml rosemary, salt, and black pepper; cook for 30 seconds.
05 -
Add red wine to the pan and deglaze, cooking until mostly evaporated. Transfer the mixture to a large bowl, stir in chopped prosciutto, and allow to cool completely.
06 -
On a lightly floured surface, roll out the puff pastry into a 25 x 25 cm square. Cut into twenty-five 5 cm squares. Arrange squares onto prepared sheet pans, spacing 5 cm apart. Prick each with a fork, leaving a 0.5 cm border.
07 -
Beat the egg with 15 ml water until smooth. Brush over the pastry squares. Add 15 ml duxelles to the center of each square and gently flatten.
08 -
Bake in the oven for 15-16 minutes, until pastry is golden and crisp. Remove from oven and transfer to a platter.
09 -
Pat beef cubes dry and season generously with salt and pepper. Heat neutral oil in the reserved pan over medium-high heat until shimmering. Sear beef on all sides, 1-2 minutes total for a browned exterior.
10 -
Add remaining 5 ml butter, remaining garlic clove, and 2.5 ml rosemary. Toss beef with garlic-rosemary butter for another 1-2 minutes, until medium rare. Transfer to a plate to halt cooking.
11 -
Top each baked pastry square with a beef cube and small dollop of Dijon sauce. Garnish with chopped chives and serve immediately.