Prepare the Chicken: Preheat your oven to 350°F (175°C). Place the chicken breasts on a lightly greased baking sheet. Bake for 20-25 minutes, or until the internal temperature reaches 165°F (74°C) and the chicken is cooked through. Let it cool slightly, then shred or dice into bite-sized pieces. (Alternatively, you can use pre-cooked rotisserie chicken – about 2.25 to 2.5 cups shredded – or prepare chicken in an Instant Pot.)
Sauté Aromatics: While chicken is baking (or if using pre-cooked chicken, proceed with this step), heat the olive oil in a small skillet over medium heat. Add the finely diced yellow onion and sauté until softened and translucent, about 3-5 minutes. Combine the cooked onion with the shredded or diced chicken.
Crisp the Flatbreads: Increase oven temperature to 400°F (205°C). Arrange the cauliflower flatbreads on a baking sheet lined with parchment paper. Bake for 8-10 minutes, until they are slightly crispy. Once cooled enough to handle, cut them into roughly 1-inch squares. Reduce oven temperature to 370°F (188°C) for the casserole.
Create the Creamy Sauce: In a medium saucepan, gently warm the green chili enchilada sauce over medium heat. Once warm, whisk in the creme fraiche and ground cumin until the creme fraiche is fully incorporated and the sauce is smooth. Remove from heat.
Assemble the Bake: Select an 8x11 inch baking dish or a similar 2-quart capacity casserole dish. Spread approximately one-third of the cut flatbread pieces evenly across the bottom of the dish. Layer with one-third of the chicken and onion mixture. Pour one-third of the creamy enchilada sauce over the chicken. Repeat these layers two more times (flatbreads, then chicken, then sauce).
Bake the Casserole: Sprinkle the shredded Monterey Jack cheese evenly over the top layer of sauce. Transfer the assembled casserole to the preheated oven (370°F / 188°C). Bake for 20-25 minutes, or until the casserole is heated through, the sauce is bubbly, and the cheese is melted and lightly golden.
Rest and Serve: Carefully remove the casserole from the oven and let it rest for about 5-10 minutes. This helps the layers set and makes serving easier. Cut into six equal portions and serve warm.