Preheat the oven to 200 C Fan, 220 C . Tear most of the mushrooms in half, or in thirds if they are large. If they are small leave them whole. Line a large baking tray with parchment.
Tip the mushrooms into the tray and mix well with the oil and salt. Make sure they are well spread out then roast for about 20-25 minutes, turning halfway. They should be golden brown and beginning to crisp around the edges. Set aside.
Meanwhile make the sauce. Put the butter, oil, onions, caraway seeds and a 1/4 tsp salt into a large saute pan. Place it on a medium heat and gently fry for about 12 minutes, stirring often, until the onions are soft and golden.
Turn the heat right down and continue to fry for 5 minutes or more untl the onions are a deep golden brown but not at all burnt or crispy. Transfer half of the onions to a plate and set aside for later.
Add the garlic to the remaining onion and fry for a minute, stirring often then remove the pan from the heat. Add the stock or water, mustard, cream , the remaining salt and plenty of pepper to the pan. Stir together then add half of the roasted mushrooms.
Return the pan to a medium heat and cook for 4 minutes then stir in the noodles. Cook for a further 3 minutes or until the sauce has thickened slightly and the noodles have cooked through.
Stir in the lemon juice and half the herbs. Divide between two bowls then top with the remaining herbs , the lemon zest and the reserved mushrooms and noodles. Spoon over the creme fraiche or soured cream if using, then finish with black pepper and more olive oil if needed. Serve immediately.