In a large heavy-based pot, melt 15g of the butter over medium heat.
55 g salted butter
Add diced onions and saute over medium-high heat until soft, then remove to a plate.
1 medium brown onion
Add diced potatoes to the pot and fill with water (enough to cover the potatoes by about 2½ cms (1 inch).
1 kg potatoes
Bring to the boil, then turn down to a simmer and continue to cook until potatoes are soft and fork tender (but not falling apart) - usually about 15 to 20 minutes.
Once potatoes are soft, spoon out approximately 1 cup of cooking water and reserve. Drain the remaining water and return the potatoes to the pot.
Add remaining butter to the potatoes and mash until to your liking.
55 g salted butter
Over low heat add back cooked onions, then add milk and salt and pepper to taste, using more milk plus some reserved cooking water to achieve your desired consistency.
350 ml whole milk
Serve once heated through.