Prepare the Mushroom Filling: In a large skillet, sauté the mushrooms, onion, and garlic in olive oil over medium heat until the mushrooms are golden brown and all the moisture has evaporated, about 10 minutes. Stir in the lentils or chickpeas (if using), thyme, and rosemary. Season with salt and pepper, and set aside to cool.
Assemble the Wellington: Preheat the oven to 400°F (200°C). Roll out the puff pastry on a floured surface and spoon the cooled mushroom filling into the center. Fold the pastry over the filling and seal the edges tightly. Brush the pastry with the beaten egg and bake for 25-30 minutes, until golden brown.
Make the Pomegranate Jus: In a small saucepan, sauté the shallots in olive oil until softened. Add the pomegranate juice and honey, and simmer until the sauce reduces by half and thickens slightly, about 15 minutes. Season to taste.
Wilt the Spinach: In a separate pan, heat olive oil over medium heat and sauté the spinach with garlic until just wilted, about 2-3 minutes. Season with salt and pepper.
Serve and Garnish: Slice the Mushroom Wellington and arrange it on the plate. Drizzle with pomegranate jus and serve with wilted spinach. Garnish with fresh herbs.