Preheat the oven to 375°F (190°C) and spray a 9 x 12 x 3-inch casserole dish with nonstick spray.
Place the 15-oz can of Hatch green chile enchilada sauce in a small saucepan and simmer until reduced to about 1 cup.
While the sauce reduces, dice the onion and green bell pepper and slice the green onions; cut cooked chicken into small dice.
Heat 1 tablespoon olive oil in a large frying pan over medium-high heat. Add the diced onion and green pepper and cook until they begin to brown, about 5 minutes.
Stir in 1 tablespoon ground cumin and cook 1–2 minutes to toast the spice, then add the frozen cauliflower rice and cook over high heat until heated through and any excess liquid has evaporated, about 4 minutes.
When the enchilada sauce is reduced to 1 cup, transfer it to a mixing bowl and stir in the drained diced green chiles; taste and add 1 tablespoon Green Tabasco Sauce if using.
Let the sauce mixture cool until warm (not hot), then stir in 3/4 cup sour cream, the sliced green onions, and the diced cooked chicken until combined.
Assemble the casserole: spread half of the cauliflower rice mixture in the bottom of the prepared dish, spoon half of the chicken/sour cream mixture over it, and sprinkle with half of the grated cheese.
Repeat the layers with the remaining cauliflower rice mixture, chicken mixture, and the remaining cheese.
Bake uncovered for 30–35 minutes, until the casserole is bubbling and the top is nicely browned.
Let the casserole rest 5–10 minutes before cutting and serving.
