Cut the potatoes in half lengthwise potatoes and place them in a medium mixing bowl.
Add the olive oil, salt, pepper, garlic powder, and mustard powder to the bowl. Toss to coat the potatoes. (Do not rinse out the bowl after you take the potatoes out.)
Use tongs to place the potatoes on the hot grill with the cut side down. Place the onion slices on the grill next to the potatoes.
Cook for 7 to 10 minutes until you can easily flip the potatoes without them tearing. Then cook for another 7 to 10 minutes.
While the potatoes are grilling, add the spicy brown mustard to the bowl you used to mix the potatoes in. Whisk the mustard into the remaining olive oil mixture in the bowl.
Remove the potatoes from the grill and place them into the bowl with the mustard. Set aside.
Transfer the grilled onions to a cutting board and chop them into small pieces. Add to the bowl with the potatoes.
Add the crumbled bacon to the potatoes and toss to coat everything in the mustard mixture. Serve warm or at room temperature.