Make the cauliflower rice first by tossing it in a wide pan with the salt, pepper, and oregano. Cook over a high heat until the rice turns slightly charred and golden.
Squeeze the lemon juice into the rice. Then, reduce the stove heat to low and melt the butter into the rice. Stir until all the rice is coated in butter, and leave it to cook a darker golden color for about 30 seconds.
Transfer the cooked rice to a serving dish and return the pan to the stove. Next, chop your chicken into bite-sized pieces and toss with the olive oil, salt, pepper, and onion powder in the pan. Cook over medium heat until the chicken is just cooked through on all sides.
Transfer the cooked chicken to the serving dish with the rice and return the pan to the stove for a third time. Melt the butter in the pan over low heat. Toss in the garlic and sliced cremini mushrooms. Cook with a lid over the pan until the mushrooms become tender and release some liquid in the pan.
Sprinkle the xanthan gum over the mushrooms. Stir the mushrooms until the xanthan dissolves. Then, pour in the chicken broth and cream. Place a lid over the pan to bring the liquid to a simmer (not a boil). Then, simmer the sauce until it reduces and thickens.
Allow the sauce to finish thickening by cooling for 3-4 minutes. Then, serve by pouring over your chicken and rice.