Preheat oven to 350F. Place potatoes in a bowl. Add butter, olive oil, garlic, salt and pepper. Toss well to combine. Spread potatoes out on a tray. Roast for 40 minutes until golden on the outside and soft inside, flipping once halfway. When potatoes are cooked, transfer to a bowl. Add onion and fresh thyme, pour over mustard and toss. Serve warm.