Grease a grill pan with cooking spray. Season chicken with salt and pepper; place in the prepared pan. Cook and stir over medium heat until chicken is no longer pink, about 7 minutes.

Heat a skillet over medium-high heat. Add onion and bell pepper slices; saute until slightly browned, about 5 minutes. Mix in diced tomatoes and cooked chicken. Cook until liquid is mostly evaporated, 2 to 3 minutes.

Microwave cauliflower rice following package instructions for 5 minutes. Let stand for 1 minute.

Carefully open the package of cauliflower rice and divide between 2 bowls. Divide the chicken and vegetable mixture evenly between the bowls. Top with Cheddar cheese, avocado, salsa, and sour cream.