Cut potatoes into 3/4-inch pieces, carrots into 1/2-inch pieces and onion into 1-inch chunks. Mince garlic.
Add vegetables, garlic, 3/4 cup broth, oil, Dijon, thyme, rosemary, paprika, 1/2 teaspoon salt and pepper to a 6-quart slow cooker. Stir and level.
Season fully thawed chicken with remaining salt and arrange in one layer over vegetables.
Cover and cook on high 2 1/2 to 3 1/2 hours, beginning checks after 2 hours 15 minutes for thin pieces.
Remove each breast as its thickest center reaches 165°F. Rest on a clean plate for 5 minutes. Continue cooking vegetables in 15-minute increments if firm.
Whisk cornstarch with remaining 1/4 cup cool broth. Stir into hot cooking liquid; cover and cook on high 10 to 15 minutes.
Slice chicken and return only 2 to 3 minutes to coat and warm. Switch off cooker, stir in lemon juice and parsley, taste and serve.