I probably don't make these the same way twice, but it's a quick way to toss together a tasty side to your main dish. As always, since I don't measure anything, these are approximate quantities of the ingredients!
Preheat oven to 400. Place scrubbed, cut potatoes and onions into gallon zip bag. While you don't want to cut the potatoes too small, they do get done faster if they're not too big. My husband chokes easily from a medical condition, so I do mine in about 1 inch chunks. At any rate, keep the potato cuts as uniform as possible so they'll get done evenly.
Stir together remainder of ingredients except for butter and pour into bag with potatoes. Close bag securely and thoroughly work oil mixture into potatoes. If the mixture is too dry, add a little more olive oil or you could even use a couple of tablespoons water. Pour into large pan (I use a jelly roll pan and it gives ample room for roasting without overcrowding. Lining the pan with aluminum foil makes cleanup a little easier if you prefer. I've also divided the potatoes into two large cast-iron skillets and this also works well.
Roast for 35-45 minutes, depending on your oven and preference of doneness, giving the potatoes one or two stirs during roasting time. Once potatoes are tender, cut butter into pieces and stir into potatoes. Turn broiler on and broil for a few minutes until nice and brown. Note: if you'd like, sample one to see if this is seasoned enough to suit you; and if needed, add additional salt/pepper to taste. Careful...they're hotter than a two-dollar pistol!
Variation: include 1/4 cup balsamic vinegar in the oil mixture, and finish with a light sprinkling of parmesan cheese before serving.