1. Heat oil over medium-high heat in a large, deep saute pan or Dutch oven, until shimmering.
2. Add carrots, then onion; saute, stirring very little at first, then more frequently, until vegetables start to turn golden brown, about 12 to 14 minutes.
3. Reduce heat to low and add butter, sugar and garlic; continue cooking until all vegetables are a rich spotty caramel color, about 15 minutes longer.
4. Add curry powder; continue to saute until fragrant, 30 seconds to 1 minute more.
5. Add broth; bring to a simmer over medium-high heat. Reduce heat to low and simmer, partially covered, until carrots are tender, about 10 to 13 minutes.
6. Using an immersion blender or traditional blender, puree until very smooth, 30 seconds to 1 minute. (If using a traditional blender, vent it either by removing the lid's pop-out center or by lifting one edge of the lid. Drape the blender canister with a kitchen towel. To 'clean' the canister, pour in a little half-and-half, blend briefly, and then add to the soup.)
7. Return to pan (or soup pot) if using traditional blender; add half-and-half to the mixture. Mixture should be soup-like, yet thick enough to float a garnish. Heat through, and ladle into bowls and serve. Garnish with chopped roasted pistachios if desired.