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In 10-inch skillet, heat butter and oil over medium-high heat. Add half of the chicken and cook, turning once, until browned, about 5 minutes. Using slotted spoon, transfer to slow cooker. Repeat with remaining half of chicken. Reduce heat to medium; add onions to skillet, and cook until softened, about 4 minutes. Add garlic and gingerroot; cook and stir 2 minutes. Add garam masala; cook 30 seconds. Add tomato paste and salt; cook and stir 2 minutes. Add chicken stock, and scrape bottom of skillet with wooden spoon. Transfer to slow cooker. Cover and cook on Low heat setting 5 to 6 hours. At last 30 minutes of cooking, stir in whipping cream.