Preheat your oven to 375°F (190°C) and adjust the rack to the middle position.

Dice the medium yellow onion and slice the carrots and celery into bite-sized pieces. Mince the garlic and chop fresh thyme leaves, discarding the stems.

In a large skillet, melt 4 tablespoons of unsalted butter over medium-high heat. Add the diced onion, sliced carrots, celery, minced garlic, thyme, kosher salt, and freshly ground black pepper. Sauté the vegetables for about 5-7 minutes, until they become tender.

Sprinkle the all-purpose flour over the sautéed vegetables and stir well, cooking for about 1 minute. Gradually whisk in 2 cups of low-sodium chicken broth and 1 cup of milk, scraping the skillet to deglaze.

Bring the mixture to a gentle simmer and let it cook for 4-5 minutes, stirring frequently. Gently stir in about 4 cups of cooked boneless chicken and 1 cup of frozen peas.

Open the package of refrigerated biscuits and arrange them in a single layer over the filling.

Bake for 20-30 minutes, checking periodically until the biscuits are golden brown and the filling is bubbling.

Remove from the oven and melt the remaining 2 tablespoons of butter. Brush over the tops of the biscuits before serving hot.