To prepare the beef Wellington, in a Dutch oven or other heavy pot, warm the canola oil until shimmering. Add the beef, season with the salt and pepper and cook until browned on all sides. Transfer to a plate.  

Pour off most of the fat in the pot. Add the onion, carrot and celery and cook, stirring occasionally, until the onion begins to turn translucent and the carrot and celery begin to soften. Add the garlic and cook, stirring occasionally, until fragrant. 

Add the tomato paste and stir until it begins to caramelize. Add the wine, stirring to scrape up the browned bits, and simmer for about 2 minutes. Return the beef to the pot and add the stock, thyme and bay leaf. Bring to a boil, then reduce the heat so the mixture simmers. Braise until the beef is fully cooked and fork-tender, about 2 hours. 

Let cool, preferably overnight in the refrigerator. The next day, shred the meat.

To prepare the sautéed mushrooms, in a large sauté pan, warm the canola oil until shimmering. Add the mushrooms and cook, stirring occasionally, until they are beginning to brown on all sides. 

Add the shallots and season with the salt and pepper. Cook, stirring occasionally, until the shallots begin to turn translucent and soften. Add the sherry and thyme and simmer until the liquid has almost evaporated. Add the cream and simmer until thickened and the mushrooms are coated with the cream. 

Remove and discard the thyme sprigs. Transfer the mushroom mixture to a food processor and pulse until the mushrooms are finely chopped but not mushy. Set aside.

To prepare the spinach, in a sauté pan over medium-high heat, warm the canola oil until shimmering. Add the garlic and stir quickly until fragrant but not toasted. 

Add the spinach and season with salt and pepper. Reduce the heat to medium and cook until the spinach wilts, adding a splash of water if needed. Set aside.

To assemble and bake the pockets, preheat the oven to 375°F.

Cut each puff pastry sheet into twelve 4-inch squares, for a total of 24 squares. Place 12 of the pastry squares on a floured cutting board. Using a pastry brush, brush the squares with the egg wash. For each pocket, put a small portion of the wilted spinach on a pastry square, then top with about 1 tablespoon of the mushrooms and 2 tablespoons of the beef. 

Cover each with one of the remaining 12 squares, sealing at the corners and along the edges. Try to press out most of the air inside the pockets as they are sealed. If desired, use a fork to crimp the edges closed for a more dramatic look. Brush the tops of the pockets with the remaining egg wash and place on a baking sheet.

Bake the pockets, turning once, until golden brown, 25 to 30 minutes. Let cool for 5 minutes and serve.