Scrub the potatoes with a brush to remove the dirt and some of the skin. Place in a saucepan filled with cold water and add a little salt. Add the stalks from the dill and parsley to the water. Bring to the boil and cover partially with a lid. Simmer until they are cooked through. Drain, Cut in half or quarter, depending on their size.
To make the dressing place the mustard in a jam jar, add the vinegar and oil. Shake vigorously. Add half the dressing to the potatoes and set to one side to cool.
Shell the peas, place in a pan of salted boiling water, simmer for a minute then drain. Run under cold water.
To make the salad: finely chop the spring onions, dill and parsley (save a few dill fronds to garnish). Place the spinach on a plate or in a shallow bowl, add the just warm potatoes, peas, herbs and spring onions. Season generously with salt and pepper and drizzle over the dressing. Toss then garnish with sorrel or micro herbs and dill fronds. Serve.