Combine the brown sugar, lime juice and chillies in a medium-sized bowl and stir to dissolve the sugar. Add the mangoes, bell peppers, cilantro and salt and pepper and gently toss. Cover and let stand at room temperature to allow flavours to meld. Brush the halibut with olive oil, and then season with salt and pepper. Grill over medium-high heat on a lightly oiled grill for 3-4 minutes per side, or until just cooked through. Gently toss salad again and divide among 4 plates. Place halibut on top or alongside the salad and garnish with lime slices.
Note: If only large halibut steaks are available, ask one of our seafood department staff to cut them in two.