Place the porcini mushrooms in a bowl, add a bit of water and rub with your fingers to remove and grit. Rinse, place in a saucepan, add half a cup water and soak for 5 minutes. Cover the saucepan (do NOT drain the water), bring to the boil, then lower the heat and simmer for 10 minutes. Set aside reserving the water.
Preheat the oven to 400 F/ 200 C/ gas mark 6. You will need a medium oven dish.
While the porcini are cooking prepare the sauce. In a large pan heat up 2 tablespoons of the oil and fry the onion gently for 3-4 minutes until softened and golden. Add the chopped regular mushrooms, increase the heat and cook for 5 minutes until they release water, stirring often.
Combine the fromage frais with the corn flour, stir until smooth then add into the pan and stir until well combined. Add the porcini mixture (including the water they cooked in), season, stir and cook for a minute until the sauce has thickened. The sauce will be quite thick.
Place the chicken strips in the baking dish in a single layer, season lightly, spoon over the mushroom sauce (ensuring all the meat has been covered), drizzle with a little oil and bake for about 20 minutes (cooking times can vary so ensure the meat is cooked through before serving, but try not to overcook it). Remove from the oven and serve immediately.
The best place to get porcini mushrooms I've ever come across is a Polish delicatessen. You'll pay a lot less than in a regular supermarket.
It is important to combine the fromage frais with the cornflour before adding into the sauce (this will ensure the sauce is smooth and the fromage frais doesn't curdle).
The mushroom sauce will be quite thick to start with but it will have the right consistency once the chicken has cooked.
Cooking times may vary depending on the size of your meat pieces.
Best served immediately. Leftovers can be refrigerated for up to 3 days. The sauce will thicken after a while so you may need to add a drop of water to loosen it when reheating the dish.