Thaw shrimp, if frozen. Peel and devein shrimp, leaving tails intact. Rinse shrimp; pat dry with paper towels. Cover and refrigerate until needed.
Lightly coat an unheated Dutch oven with nonstick cooking spray. Preheat over medium heat. Add onion and garlic. Cook until onion is tender.
Remove skin from the chicken thighs. Add chicken thighs, broth, dried thyme (if using), salt, saffron, and cayenne pepper to Dutch oven. Bring to boiling; reduce heat. Simmer, covered, for 15 minutes.
Stir in uncooked rice. Return to boiling; reduce heat. Simmer, covered, about 15 minutes more or until rice is almost tender. Stir shrimp, sweet pepper, peas, tomato, and fresh thyme (if using) into the rice mixture. Simmer, covered, for 5 to 8 minutes more or until the rice and chicken are tender and shrimp turn opaque. Makes 6 servings.