Preheat the oven to 350°F. Lightly grease your chosen baking dishes: either 3 9-inch pie plates, 3 8x8-inch glass baking dishes, or 1 9x13-inch and 1 8x8-inch baking dish.
In a large sauté pan over medium heat, melt the butter. Add the mushrooms, onions, and bell pepper. Cook, stirring occasionally, until the vegetables are tender and most of the butter is absorbed, about 5-7 minutes. Remove from heat and set aside
Steam the frozen cauliflower according to package directions until just tender, typically 5-6 minutes. Drain excess liquid.
In a large bowl, combine the cooked chicken, drained peas, drained tomatoes, cubed cheese, steamed cauliflower, and sautéed veggie mixture. Season lightly with salt.
Divide the mixture evenly among your baking dishes, smoothing the top with a spatula.
Bake uncovered for 30 minutes, or until the casserole is bubbly and the cheese is fully melted.
Remove from oven and let cool for 5 minutes before serving.