Preheat the oven to 350 degrees F.
In a pan with a tight-fitting lid that can go from stovetop to oven, heat the oil over medium heat.
Add the mushrooms and onions and sauté for 8-10 minutes or until they are browned and beginning to caramelize, stirring occasionally. Stir in the garlic and cook, stirring, for 30-60 seconds or until you can smell the garlic but it is not turning brown.
Add the rice and stir until coated. Sprinkle with the salt, pepper, and thyme.
Carefully, pour in the broth, increase the heat, and bring to a boil.
As soon as the mixture boils, remove from the heat, cover, and bake for 45 minutes or until the broth is absorbed.
Remove the pan from the oven and allow it to sit, covered, for 10 minutes before serving. If you’d like to top with the optional cheese, quickly sprinkle 1/4 to 1/2 cup overtop, and replace the lid.