Preheat your oven to 375°F (190°C).
In a large Dutch oven over medium-high heat, melt 2 tablespoons of butter. Sear the chicken thighs for 2-3 minutes per side until lightly browned.
Sprinkle the ranch seasoning and au jus mix over the chicken. Add the sliced pepperoncini peppers and pour in 1/2 cup of pepperoncini juice. Bring to a simmer, then reduce heat to low, cover, and cook for 25-30 minutes until the chicken is fall-apart tender.
While the chicken cooks, bring a large pot of salted water to a boil. Cook the elbow macaroni for 1-2 minutes less than the package directions for al dente. Drain well and set aside.
Remove the cooked chicken to a cutting board and shred it using two forks. Strain the remaining cooking liquid through a fine-mesh sieve into a measuring cup and set aside. You should have about 1 to 1.5 cups.
Wipe out the Dutch oven and melt the remaining 4 tablespoons of butter over medium heat. Whisk in the flour and cook for 1 minute, stirring constantly, to form a roux.
Slowly whisk in the reserved chicken cooking liquid until smooth. Then, gradually whisk in the whole milk. Continue cooking, stirring frequently, for 5-7 minutes until the sauce thickens enough to coat the back of a spoon.
Reduce the heat to low. Add the cubed cream cheese and stir until completely melted and the sauce is smooth. Remove the pot from the heat.
Add the shredded sharp cheddar and Monterey Jack cheeses in handfuls, stirring until each addition is fully melted before adding the next. Season with salt and pepper to taste, being mindful of the saltiness from the seasoning packets.
Add the cooked pasta and shredded chicken to the cheese sauce. Stir gently until everything is thoroughly combined.
Pour the mixture into a 9x13-inch baking dish and spread into an even layer. Bake for 20-25 minutes, until the sauce is bubbly and the top is lightly golden.
Let the mac and cheese rest for 10 minutes before serving to allow the sauce to set.