Place the potatoes and onion on a baking sheet. Drizzle them with the olive oil then season with sea salt and freshly cracked pepper, to taste; toss to coat evenly.
Place into the oven and roast for 10 minutes; stir the potatoes and onion around with a spatula then return to the oven and roast for another 5-7 minutes, or until the potatoes are tender.
Add the butter and minced garlic to the baking sheet and toss to evenly coat the potatoes an onion; return to the oven to roast for 1-2 minutes.
Remove from the oven and top with parsley. Taste & re-season with sea salt & freshly cracked pepper, if needed. Serve immediately. Enjoy.