Season lamb with salt and pepper. Brown in oil over medium-high heat in two batches, 6 to 8 minutes each.
Transfer to a 6-quart slow cooker; add onion, carrots, parsnips, and garlic.
Stir tomato paste in the hot skillet for 1 minute. Add broth and wine; scrape up browned bits.
Pour into cooker; add rosemary and bay leaf. Cover.
Cook on low for 7 1/2 to 8 hours until lamb yields easily to a fork and parsnips are tender.
Discard bay leaf; skim excess fat. Stir in peas.
Cover; cook on high for 8 minutes. Add lemon juice; check salt.