Heat your oven to 375 degrees. Warm the olive oil in a large skillet over medium-high heat. Add the ground turkey, breaking it up with a spoon. Cook until it is no longer pink and starting to brown at the edges, about 6 to 7 minutes. Transfer it to a plate and set aside.

In the same skillet, add the diced onion and sliced mushrooms. Cook over medium heat, stirring occasionally, until the mushrooms have released their liquid and that liquid has evaporated, about 8 minutes. The mushrooms should look golden and reduced. Add the garlic and tomato paste and cook 1 more minute, stirring constantly, until fragrant.

Return the cooked turkey to the skillet. Add the rinsed pearl barley, chicken broth, Worcestershire sauce, rosemary, thyme, salt, and pepper. Stir everything together and taste the liquid. Adjust salt if needed. Bring to a simmer over medium heat for about 2 minutes, just to get things going.

Pour the entire mixture into a 2 1/2 quart baking dish and spread it out evenly. Cover the dish very tightly with two layers of aluminum foil. A tight seal is important here because the barley needs the trapped steam to cook through properly.

Bake covered for 50 minutes. Remove the foil carefully and check the barley. It should be plump and tender with just a small amount of liquid remaining. If it still has a bite in the center, cover and bake another 10 minutes.

Once the barley is fully tender, remove the foil, sprinkle Parmesan over the top if using, and return the dish to the oven uncovered for 5 minutes. Let the casserole rest for 5 minutes off the heat before scooping. This allows the liquid to be fully absorbed and the texture to set up.