In a large cast-iron skillet, heat 3 tablespoons olive oil over medium-high heat. Add the mushrooms to the skillet, season with salt and pepper, and cook for 6-7 minutes or until they are soft, stirring. Add the tomatoes and rosemary and cook for an additional 3 minutes, stirring. Remove from the heat and set aside.
Add the eggs to a medium bowl. Season with salt and pepper and whisk until combined. Set aside.
In a medium cast-iron skillet, heat the additional 2 tablespoons olive oil over medium-high heat. Pour the eggs into the skillet and cook for 4-5 minutes or until just set. Spoon the mushrooms and tomatoes evenly on one side of the omelet and fold the other side over.
Transfer the omelet to a large plate. Serve immediately.