Toss chicken with flour, 0.5 tsp salt, 0.25 tsp pepper, and paprika until lightly coated.
Heat olive oil in a skillet over medium-high. Sear chicken in batches until lightly browned, 3–4 minutes. Transfer to slow cooker.
Add onion, celery, carrots, potatoes, and garlic to the slow cooker, spreading evenly.
Whisk broth with tomato paste, thyme, rosemary, remaining salt, and remaining pepper. Pour over the mixture.
Cover and cook on LOW for 6–7 hours or on HIGH for 3–4 hours, until chicken is tender and potatoes are soft.
Stir in peas and parsley. Cook uncovered 10–15 minutes to heat through and slightly thicken.
Finish with lemon juice if using. Taste and adjust seasoning as needed.
Let the stew rest 5 minutes before serving so the broth settles.