Heat oil in a very large skillet over medium heat. Add onion and carrot; cook, stirring occasionally, until softened, 3 to 4 minutes. Add bell pepper and jalapeño; cook, stirring occasionally, until softened, about 3 minutes.
Add curry paste, ginger, and garlic; cook and stir until fragrant, about 30 seconds. Add coconut milk, pumpkin, maple syrup, and fish sauce; bring to a boil over high heat. Reduce heat to medium and simmer, uncovered, stirring occasionally, until thickened, about 5 minutes.
Arrange halibut in skillet. Simmer, covered, 3 to 5 minutes or until opaque and an instant-read thermometer inserted into thickest portion registers 145 degrees F (63 degrees C). Top with cilantro. Serve with rice and lime wedges. Garnish with pepitas.