Preheat oven to 375 degrees.
Mix 2 tbsp olive oil with potatoes and onions.
In separate bowl, mix lime juice, syrup, and chili powder.
Rub 1 remaining tbsp of oil on the roast, and then rub the lime juice mixture on the roast, making sure to coat it well.
Put the roast upside down (fat side up) into a large, 2-3 inch deep baking pan and then surround the potatoes and onions all around the sides of the roast in the pan.
Cook the roast on middle rack of oven for approximately 40 minutes for medium rare. (Figure 20 minutes per pound for medium rare, 30 minutes per pound for medium-well).
Remove pan from oven and cover tightly with aluminum foil, allowing to sit for 5 minutes.
Slice immediately, transfer potatoes and cooking juices together into a bowl and serve.