Cook the pasta according to packet instructions, drain and keep warm in the pan.
Meanwhile, finely chop the garlic and spring onion (green parts too). Heat the oil in a pan and gently fry them until translucent.
Chop the mushrooms and add to the pan, then gently sauté for 8-10 minutes, until soft and the water has evaporated. Add the spinach, cover and gently steam until it has wilted.
Remove the pan from the heat and stir in the kefir or yoghurt. Season to taste with salt & pepper. Add the lemon zest and chilli flakes, if used.
Toss the pasta with the creamy mushroom sauce, then warm it up if necessary and serve immediately.