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