Slice and chop the mushrooms and onions. Sauté them in butter or oil until softened.
In a large bowl, combine the sautéed onions and mushrooms, yellow peppers, spinach, bread, and chicken. Mix everything together thoroughly.
Pour the mixture into a glass bread pan, ensuring it is evenly spread out.
In a mixing cup, combine the eggs and half and half. Stir vigorously until well combined.
Pour the egg and half and half mixture over the top of the bread mixture, making sure that all the bread is evenly covered.
Arrange slices of butter on top of the breading, along with the tomato slices.
Cover the pan with foil and refrigerate overnight, or for at least 2 hours, allowing the bread to soak up the egg and half and half mixture.
Preheat the oven to 375°F. Remove the foil from the pan and place the strata into the preheating oven to come to room temperature.
Bake for 30-40 minutes or until the sides are bubbling and the top has toasted to a golden brown.
Once ready, remove from the oven and serve immediately. Enjoy!