Start by reducing the red wine in a pot until 1/4 original volume. Then add the bone stock and reduce again until roughly 1/6 volume. Add heavy cream and reduce once more for 8 minutes. Season with salt and pepper to taste.
While all of that is reducing we start the duxelles. Heat butter in a skillet over medium-high heat. After butter melts add onions and garlic and sauté for 3 minutes. Next add mushroom pieces, salt/pepper, and thyme. Reduce the heat to medium or medium-low and simmer until all liquids have evaporated. (10 to 12 minutes). Add some wine and cook until evaporated, usually around 4-5 minutes.
Lightly flour a cutting board and rolling pin. Lay your puff pastry down and roll until thin (thickness of a tortilla). Next, take a fork and poke several holes along the surface as to prevent it from rising too high. Cut into 2-inch by 2-inch squares and brush with either egg wash or butter. Place in oven until fully cooked and golden brown.
Heat a pan with olive oil to medium-high.
Break down tenderloin into roughly 1 inch by 1 inch cubes season with salt and pepper then sear all sides until the desired temperature. Set aside to rest.
Now we can put the dish together.
Place the puff pastry onto the desired platter, then spoon down the duxelles onto the base. Next, we add the seared tenderloin followed by a drizzle of the glaze. Garnish with some fresh parsley and serve warm.