Preheat the oven to 375℉.
Season both sides of the chicken with salt and pepper. Add the cassava flour to a shallow bowl and dredge each chicken breast in the flour, shaking off any excess. Set aside.
In a large deep oven-safe skillet, heat the oil over medium-high heat. Add the chicken and sear until golden brown on both sides, about 3 minutes per side. Remove from the skillet and set aside. The chicken will not be cooked through as it will finish cooking in the oven.
Reduce the heat beneath the skillet to medium and add the garlic, bell pepper, onion, and zucchini. Sauté, stirring, until the veggies begin to soften, about 2 minutes. Add the black beans and stir to combine then spread the veggie mixture into a single layer.
Nestle the chicken back into the skillet then pour the enchilada sauce over the chicken and veggies. Top each chicken breast with ¼ cup of cheese. Top with jalapeño slices, if using.
Place into the oven and bake, uncovered, until the chicken is cooked through and cheese is melted and bubbly, about 10 minutes.
Remove from the oven and top with fresh cilantro. Serve as-is or with Cilantro Lime Rice, if desired.