Place hazelnuts in a food processor; process until smooth (about 2 1/2 minutes), scraping sides of bowl once.
Heat a large Dutch oven coated with cooking spray over high heat. Add lamb; cook 5 minutes, browning on all sides. Reduce heat to medium-high. Add onion and next 5 ingredients (onion through garlic); sauté 7 minutes or until onion is lightly browned.
Add hazelnut butter and broth to pan; stir well, scraping pan to loosen browned bits. Stir in turnips, potatoes, carrots, dates, and 2 cups water; bring to a boil.
Reduce heat to medium. Cover and simmer 15 minutes. Uncover and simmer 1 hour and 20 minutes, or until lamb is tender and sauce has thickened. Stir in peas; simmer 5 minutes.