Whisk the oil, mustard, rosemary, and salt in a large bowl. Set aside.
1 tablespoon olive oil, 1 tablespoon grainy mustard, 1 teaspoon dried rosemary, ½ teaspoon kosher salt
Scrub and prep the potatoes. If they are small, just quarter them; larger ones should be cut in about 1-inch chunks.
1.5 pounds of red-skin potatoes
Put the potatoes in the bowl with the mustard mixture and toss until everything is coated.
Line a large cookie sheet with aluminum foil and spray with non-stick spray (they will stick). Dump the potatoes on the cookie sheet and spread them out.
Roast them for 45 minutes tossing 3 times.
I set a timer for 15 minutes, toss, reset for 15 minutes, toss, reset for 15 minutes and done!