Cut mushrooms into even, bite-sized pieces. Drizzle a little oil into a roasting pan, add the mushrooms, and toss together thoroughly to coat. Roast for 10 to 15 minutes, or until mushrooms are tender but have not yet begun to shrivel. Set aside.
Put tea bags into a bowl, cover with 2 1/2 cups boiling water, and let stand to infuse for 4 minutes. Remove and discard tea bags and set the liquid tea aside.
Melt butter in a large saucepan over medium heat, add onions and cook, stirring for 5 minutes, or until soft. Add garlic and paprika and cook for another 2 to 3 minutes. Add corn flour and stir together well. Stir in the tea and heavy cream. Then add mushrooms, mustard and tomato paste. Bring to a simmer and cook gently for 15 minutes, stirring occasionally, until sauce has begun to reduce and thicken slightly.
Add sour cream and cook for another 5 minutes, or until stroganoff is thick and creamy. Season with salt and pepper to taste. Divide between serving plates, scatter with chopped dill and serve with Walnut and Leek Pilaf.
Serves 8 to 10 as a side dish.
Note: From "Mildreds: The Cookbook," by Mitchell Beazley.