Place the butter in the base of a medium-sized, heavy casserole or heavy-based pot with a tight-fitting lid. Place the pot over a medium-low heat to melt the butter.

Add the potatoes, salt and pepper and toss well to coat all potatoes in butter. Cover the pot and cook over a medium-low heat for 25-30 minutes until the potatoes are fork-tender. Without removing the lid, shake the pot occasionally to prevent the potatoes on the bottom from burning. (Alternatively, place the covered heatproof pot in a preheated 190ºC oven and cook for 40-50 minutes until the potatoes are fork-tender and beginning to brown.)

Once cooked, turn off the heat, add both quantities of peas and leave to steam for a further 5 minutes or until the peas are cooked. Add the herbs and toss. Serve hot.