Melt the butter in a large skillet over medium high heat. When it begins to foam, add the onion, celery, and carrots. Add a pinch of salt then reduce heat to medium low. Stir and cook until soft. Add the garlic and cook for 1 minute, stirring constantly.
Sprinkle the thyme, rosemary, and sage onto the veggies. Let the spices bloom by stirring constantly for a minute.
Increase the heat to medium, then sprinkle on the flour. Whisk until the flour seems cooked, about 3 minutes.
Whisk in the chicken stock first, and then the whole milk. Increase the temperature a tiny bit and let the milk heat up. Whisk constantly for a couple of minutes, until the sauce has thickened. Lower the heat to medium low, add the diced roasted red peppers, shredded meat, frozen peas, salt, and white pepper to taste. Stir and cook until heated through, roughly 5 minutes.
Serve warm over cooked rice or the carb of your choice with chopped parsley as a garnish.