Pat beef cubes dry with paper towel. Season evenly with 1 tsp salt and 1 tsp black pepper. Heat a heavy skillet over high heat, add 2 tbs olive oil and 1 tbs butter until shimmering. Sear beef cubes 30–45 seconds per side until browned but not cooked through. Transfer to a plate and let rest in the refrigerator 10 minutes to cool slightly.

Make Mushroom Duxelles

In the same skillet over medium heat, add 1 tbs minced shallot and 1 tsp minced garlic; cook 30 seconds until fragrant. Add 1 cup finely chopped cremini mushrooms and 1 tsp chopped thyme. Cook, stirring frequently, until mushrooms release moisture and it evaporates, about 6–8 minutes. Add 2 tbs dry white wine (or stock) and cook until nearly dry. Remove from heat and cool completely.

Assemble Bites

Preheat oven to 400°F (200°C). Lightly dust a work surface with 1 tsp flour and roll out 1 sheet thawed puff pastry to smooth seams. Cut pastry into 12 roughly 3x3-inch squares. On each square place about 1 tsp cooled mushroom duxelles, then one seared beef cube. Optionally tuck a thin slice (about 1/4 oz) of gouda or cheddar under or over the beef cube for extra richness. Brush pastry edges with beaten egg and fold to enclose the beef, sealing edges; place seam-side down on a parchment-lined baking sheet. Lightly brush tops with egg wash and chill 5–10 minutes in refrigerator to set.

Bake

Bake in the preheated oven 12–16 minutes until pastry is golden brown and an instant-read thermometer inserted into beef reads 120–125°F for medium-rare. Remove from oven and let rest 3 minutes. Keep warm while finishing sauce.

Demi-glace & Finishing

Warm 3 tbs demi-glace in a small saucepan over low heat. If desired, add 1 tbs red wine and whisk to combine; heat until gently simmering, then reduce heat so sauce is glossy and slightly thickened (about 2–3 minutes). Taste and adjust seasoning with a pinch of sea salt and freshly ground black pepper.

Spoon a small amount of warm demi-glace over each Beef Wellington bite or serve demi-glace in a small dipping bowl for guests. Sprinkle each bite with a small pinch of chopped fresh parsley for color.

Cheese & Meat Platter Assembly

On a large serving board arrange the 12 Beef Wellington bites slightly off-center. Fan and nestle the sliced gouda and cheddar in small stacks; place crumbled gorgonzola in a small ramekin or scatter small clusters; place dollops or a small log slice of goat cheese with a small spreading knife.

Fold or roll 4 oz prosciutto and 4 oz soppressata into compact rosettes and tuck them between cheese stacks. Arrange 40 gourmet crackers around cheeses and meats so each item is easy to reach.

Finish the board by drizzling a small amount of extra demi-glace near the Beef Wellington bites or place additional demi-glace in a small dipping dish. Sprinkle remaining 1 tbs chopped parsley over the board and finish with a light sprinkle of sea salt and cracked black pepper over the goat cheese and crackers as desired.