Preheat the oven to 390 degree Fahrenheit. Brush two 8" inch baking dishes or a 9"x13" inch baking dish with oil. Put them in the oven to preheat too.
Heat a pan in medium. If you're using chicken strips, brown the chicken strips. Remove them from the pan and keep it aside. If not, proceed to the next step.
Melt the butter in the same pan and add onion and minced garlic. When the onion turns translucent add the sliced mushrooms.
Cook the mushrooms till they are lightly browned. Sprinkle a pinch of salt. Remove from the stove and keep it aside.
In a large bowl, add the eggs, salt and whisk well. Add the ground pepper and red chili flakes. Next the cheese, cooled mushrooms and torn chicken strips. Fold in the spinach leaves.
Pour in the baking dish and bake for 25 minutes or until the eggs are set in the middle.
Let it cool and cut into slices and serve with salad and salsa on top.