Melt 1 tablespoon of light butter in a large Dutch oven or heavy-bottomed stockpot over medium-high heat. Toss in the diced onion, chopped carrots, and celery. Sauté for 3 minutes until they begin to soften. Stir in the minced garlic and cook for exactly 1 minute longer until highly fragrant.

Drop the remaining 2 tablespoons of light butter into the pot with the vegetables, letting it melt completely. Sprinkle the ⅓ cup of all-purpose flour over the top. Stir continuously for 2 minutes to cook out the raw flour taste; the mixture will look thick and pasty.

Slowly stream in the 4 cups of chicken broth while whisking vigorously to break up any flour lumps. Add the diced potatoes, bay leaf, chicken bouillon, salt, black pepper, oregano, basil, thyme, and sage. Bring to a boil, then lower the heat to a gentle simmer. Cover and cook for 15–20 minutes until the potatoes are fork-tender.

Remove the lid and fold in the 3 cups of shredded rotisserie chicken. Let the soup simmer uncovered for another 5 minutes to warm the chicken thoroughly and allow it to absorb the seasoned herb broth.

In a small bowl, whisk the 2 tablespoons of cornstarch into the 2 cups of cold half-and-half until completely smooth. Pour this slurry into the bubbling soup, stirring constantly. Add the frozen peas. Simmer and stir frequently for 5–8 minutes until the liquid transforms into a rich, velvety gravy. Remove from the heat and discard the bay leaf.