Instructions

1. Pour the hot water over the dried mushroom pieces and leave them half an hour. Mushrooms rehydrate slowly and a piece that goes into the pot still hard will stay hard however long the soup simmers.
2. Lift the mushrooms out with a slotted spoon and keep the soaking water. That liquid is where all the flavour went, and pouring it away is the most common mistake in this kind of soup.
3. Strain the soaking water through a paper towel or a fine cloth. Dried mushrooms carry grit that settles to the bottom of the bowl, and the towel is the only reliable way to leave it behind.
4. Put the mushrooms, penne, dried tomato slices, onion flakes, garlic, thyme, basil, parsley and pepper flakes into a large pan with the strained liquid and enough extra water to cover everything well. The pasta will take up far more than looks reasonable.
5. Bring it up to a boil, then drop it to a simmer. Boiled hard, the penne breaks up and the starch it sheds makes the soup cloudy and gluey.
6. It is ready when the pasta has no chalky line at its centre and the tomato slices have softened into the liquid rather than floating on it. Top up with hot water rather than cold if it gets too thick, so the simmer never stops.
7. Hold the parsley and basil back until the last few minutes if you want them to taste of anything. Dried leaf herbs give up their flavour quickly and then go flat.
8. Measure the dry ingredients into a sealed jar or bag as a made-up mix and it will sit in the cupboard for six months. The mushrooms keep longest and the dried herbs fade first, so write the date on the lid.