In a small bowl, whisk the Dijon mustard, sour cream, mayonnaise, garlic, and a pinch of salt together. Taste and add more salt, if necessary. Cover the bowl and refrigerate for later.

Preheat oven to 400°F. Line two sheet pans with parchment paper.

Add the mushrooms, shallots, and 2 cloves of garlic cloves to the bowl of a food processor. Pulse until the mixture is finely chopped, scraping down the sides if necessary.

To make the duxelles, melt 1 Tablespoon of the butter in a large pan over medium heat. Add the mushroom-shallot mixture and cook until the veggies are very soft and the liquid is evaporated (it will almost be a paste). Add 1 teaspoon of rosemary and salt and pepper to taste and cook for about 30 seconds.

Then, deglaze the pan with the red wine. Cook until the wine is mostly evaporated. Transfer the mixture to a large bowl and stir in the chopped prosciutto. Cool completely. Then, wipe out the pan and set it aside for later.

Next, roll the puff pastry on a lightly floured surface until it’s 10-by-10 inches. Cut into 25 two-inch squares. Place the squares on the prepared sheet pans about 2 inches apart and prick them with the tines of a fork, leaving about 1/4-inch border.

Beat the egg with 1 Tablespoon of water until it's smooth. Brush the egg wash over the puff pastry squares. Then, spoon about 1 Tablespoon of duxelles into the center of each square. Use a spoon to gently flatten the duxelles. Then, bake 15-16 minutes, until the puff pastry is golden-brown.

Meanwhile, pat the beef cubes dry and generously season them with salt and pepper, to taste. Using the same pan that you made the duxelles in, heat the neutral oil over medium-high heat until it's shimmering. Add the beef cubes and cook until they’re seared all over, about 1-2 minutes.

Add the remaining 1/2 Tablespoon of butter, chopped garlic clove, and 1/2 teaspoon of rosemary to the pan, and toss the cubes with the garlic-rosemary butter until the beef cubes are medium rare, another 1-2 minutes. Transfer to a plate to prevent further cooking.

Transfer the puff pastry squares to a serving platter and add a beef cube to the center of each pastry. Add a small dollop of Dijon sauce on top of each beef Wellington and garnish with chives. Enjoy!