For the flour blend: Whisk together the rice flour, tapioca flour, potato starch, xanthan gum and salt in a large bowl.
For the topping: In a small bowl, stir together the cereal, mustard and oil.
For the green beans: Preheat the oven to 350 degrees F. Grease a 9-by-13-inch baking dish. Heat the oil in a large skillet over medium-high heat. Add the mushrooms and bell peppers, and cook, stirring occasionally, until golden. Season with salt and pepper. Sprinkle the flour blend over the vegetables, and cook, stirring, for about 1 minute. Stir in the broth; bring to a simmer, and cook until thickened, about 4 minutes.
Add the green beans to the prepared baking dish. Top evenly with the mushroom mixture. Scatter the topping over the top, and drizzle with olive oil; bake until golden and bubbling, about 25 minutes.