Cut up the onions into thin round slices i.e. at right-angles to the ends, then slice the mushrooms and garlic.
Put a couple of Tbsps of water into a biggish casserole and add the onion slices. Cover, but stir every couple of minutes, then after five minutes add the garlic.
Cover again, but continue to stir regularly for another 5-6 minutes after which you pour in the mushrooms and re-cover.
When the mushrooms have softened and reduced, add the water gradually while stirring in the misto paste. Pop in the noodles, turn up the heat until just boiling, then cover and leave simmering for about 10 minutes before serving.