Select saute setting on a 6-qt. electric pressure cooker. Adjust for medium heat; add oil. When oil is hot, cook and stir onion until crisp-tender, 5-7 minutes. Add rice and garlic; cook and stir until rice is coated, 1-2 minutes. Add stock; cook 1 minute, stirring to loosen browned bits from pan. Press cancel.
Stir in pumpkin, curry powder, rosemary, salt and pepper. Lock lid; close pressure-release valve. Adjust to pressure-cook on high for 7 minutes. Quick-release pressure. If desired, serve with additional minced rosemary.
Slow cooker option: Heat oil in a 6-quart slow cooker on high heat until hot. Add rice; stir to coat. Stir in remaining ingredients. Cook, covered, on low until rice is tender, 3-4 hours, stirring halfway.