Preheat your oven to 350°F (175°C).
In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add the diced onion and minced garlic, sauté for about 3 minutes until softened.
Add the sliced mushrooms to the skillet and cook for another 5 minutes until they are browned and have released their moisture.
Season the chicken thighs with salt, pepper, thyme, and oregano. Push the vegetables to the side of the skillet and add the seasoned chicken thighs.
Cook the chicken for about 5-7 minutes per side until they are golden brown and cooked through. Remove from heat.
In a separate bowl, combine the heavy cream and chicken broth and stir until well blended.
In your casserole dish, layer the cooked chicken and mushroom mixture evenly. Pour the cream and broth mixture over the top, then sprinkle the shredded cheddar cheese over everything.
Cover the casserole dish with foil and bake in the preheated oven for 25 minutes.
After 25 minutes, remove the foil and bake for an additional 10 minutes or until the cheese is bubbly and golden.
Remove from the oven and let it cool for a few minutes. If desired, garnish with fresh spinach before serving.