Season thawed chicken with 3/4 teaspoon kosher salt, 1/4 teaspoon pepper, garlic powder and thyme.
Add oil and onion to a 6-quart slow cooker. Add chicken, garlic and 1 1/2 cups broth.
Cover and cook until every thickest thigh reaches 165°F, about 2 to 3 hours on high or 4 to 5 hours on low. Transfer chicken to a clean covered plate.
Switch to high. Stir in converted rice and the remaining 1 1/4 cups broth, heated until steaming. Cover 35 minutes.
Stir once and cook 10 to 25 minutes more, until rice is almost tender; add 1/4 cup boiling broth if dry.
Spread broccoli on top. Cover 15 to 25 minutes, until broccoli is crisp-tender and rice is tender.
Switch off. Shred chicken and return it. Temper cheddar, yogurt and Dijon with hot rice mixture, then fold in. Add lemon and remaining seasoning.
Rest covered off heat for 5 minutes and serve promptly.