Heat olive oil in a large pot over medium-high heat. Add onion, celery, and carrots, along with a couple pinches of salt and pepper. Cook for 8 minutes, stirring frequently.
Then add mushrooms, sage, thyme, garlic powder, sliced garlic, and a large pinch of salt and pepper.
Cook for about 5 minutes, stirring occasionally.
Add balsamic vinegar and a splash of veggie broth to deglaze the pot, scraping all the yummy bits off the bottom.
Dissolve cornstarch in 3 tablespoons of COLD water and add the slurry to the pot as well. Add a couple pinches of salt and pepper.
Stir to combine and bring to a simmer over medium high heat.
Cover pot and reduce heat to low. Simmer, covered for 30-35 minutes or until the rice is almost cooked through. Stir every 10 minutes or so to make sure the rice doesn't stick to the bottom of the pot.
Remove lid and simmer for another 10-15 minutes or until the rice is cooked through, and the soup has thickened.
Stir in fresh parsley, season to taste with salt and pepper and enjoy!