Mushroom Wellington

Name: Mushroom Wellington
Ingredients: 6 medium or 4 large portabello mushrooms, stalks trimmed and cleaned; 3 onions, peeled and chopped; 2 tablespoons of olive oil; 2 tablespoons of butter; 300g of baby spinach; 4 sprigs of thyme; 1 sheet of frozen puff pastry, partially thawed; 1 egg, beaten; 1 tablespoon of dijon mustard; Salt and pepper to taste