Place potatoes in cold salted water and bring to the boil. Cook potatoes until tender.
Drain, cool and cut into slices.
Chop the asparagus and chilli (finely).
Heat a skillet, brush it with a little butter and sear the potatoes on each side. Then set them aside and repeat the process with the asparagus, chilli and then the red onion.
Combine the vegetables and toss through the spinach leaves and thyme. Pour over the butter and the lemon juice.