Place potatoes, carrots, mushrooms and onion in a 6-quart slow cooker. Arrange chicken over the vegetables and add broth, thyme, salt and pepper.
Cover and cook on low for 5 to 6 hours until potatoes are tender and chicken reaches 165°F and separates easily with a fork.
Mix cornstarch with cold water. Stir the slurry and frozen peas into the stew and turn the cooker to high.
Cover and cook for 20 to 25 minutes until the gravy bubbles lightly and thickens, stirring once after 15 minutes.
Stir in cream and Dijon and heat covered for 5 minutes more. Divide among 6 bowls without shredding all the chicken into the gravy.