Cook the spelt in a large pan of boiling salted water for 10-12 minutes, until al dente. Meanwhile, for the dressing, finely chop most of the herbs and garlic (you can do this in a food processor).
Transfer to a bowl and whisk in the mustard, vinegar, yogurt, 1 tbsp water and 2 tbsp oil until thoroughly combined; season. Drain the spelt and transfer to a bowl. While hot, add the dressing and stir to combine; set aside.
Put the remaining 2 tbsp oil in a large pan over a medium-high heat. Stir-fry the kalettes for 2-3 minutes until vibrant green and slightly softened; season.
Combine the spelt, kalettes and peppers and arrange on a serving dish. Scatter over the walnuts and remaining herbs, then serve straight away.