1. Preheat your oven to 175°C or 350°F.

2. Cut the chicken into bite-sized pieces and season with salt and pepper. Cook the chicken in a large skillet over medium-high heat until browned, which should take around 5 minutes. Remove the chicken from the heat and set it aside for now.

3. In the same skillet, add some butter and sauté the onions until they're softened, about 3 minutes. Next, add the mushrooms and cook until they release their moisture and start to brown, which should take around 5 minutes.

4. Add the Brussels sprouts, red bell pepper, and soy sauce to the skillet. Let it cook for an additional 2 minutes, until the vegetables are tender-crisp.

5. In a large bowl, whisk together 250ml or 1 cup of heavy cream, 125ml or 1/2 cup of chicken broth, 30g or 2 tablespoons of Dijon mustard, and 30g or 2 tablespoons of flour. Add the cooked chicken, mushroom mixture, and 5ml or 1 teaspoon of sambal oelek to the bowl. Stir everything together until it's well combined.

6. Transfer the mixture to a 23x33cm or 9x13 inch baking dish and bake it in the oven for 25-30 minutes, or until the sauce is bubbly and the casserole is hot throughout.

7. Serve the casserole hot, garnished with chopped fresh herbs if you like.