Heat the large pot over medium heat and add the olive oil. Once shimmering, add the diced onion and chopped celery, sautéing until they soften and become fragrant, about 5 minutes.
Add the minced garlic and chopped sage to the pot, cooking for another minute until the garlic is fragrant and slightly golden. Stir frequently to prevent sticking.
Pour in the chopped seasonal root vegetables and stir to coat them with the aromatic mixture. Let them cook for 3–4 minutes to start softening and absorb flavors.
Pour in the sage-infused chicken broth and bring the mixture to a gentle simmer. Cover the pot and let it cook for about 30 minutes, or until the vegetables are tender and the flavors meld together.
Uncover the pot and check the seasoning, adding salt and pepper to taste. If you prefer a thicker consistency, stir in a slurry of a teaspoon of cornstarch mixed with water and cook for another 5 minutes until thickened.
Once the vegetables are tender and the stew has thickened to your liking, remove from heat. Serve hot, garnished with a fresh sprig of sage if desired, and enjoy the comforting aroma and hearty texture.