Preheat your oven to 400°F (200°C). Dice the russet potatoes into uniform, one-inch pieces. Toss the diced potatoes with olive oil, ranch seasoning mix, salt, and pepper on a large sheet pan.

Spread the potatoes into a single layer. Roast for 20 to 25 minutes, or until they are fork-tender and beginning to brown nicely. Once cooked, transfer the roasted potatoes to a greased 9x13 inch baking dish.

While the potatoes roast, prepare the chicken. If using raw chicken breast, cook it (boil, bake, or pan-fry) and shred or cube it. In a large bowl, combine the cooked shredded chicken with the softened cream cheese, sour cream (or Greek yogurt), and buffalo sauce. Stir until the mixture is completely combined and the chicken is fully coated.

Pour the creamy buffalo chicken mixture evenly over the layer of roasted potatoes in the baking dish. Gently spread the chicken mixture so it covers all the potatoes, but try not to mash the potatoes too much.

In a separate small bowl, gently combine the shredded cheddar cheese and Monterey Jack cheese. Sprinkle the cheese blend evenly over the buffalo chicken layer. Distribute the crumbled bacon bits over the cheese, if using.

Return the casserole dish to the 400°F (200°C) oven and bake for 15 to 20 minutes. The casserole is done when the filling is hot and bubbly and the cheese topping is melted and slightly golden.

Remove the casserole from the oven and let it rest for 5 minutes. Drizzle generously with ranch dressing or blue cheese dressing, then scatter the sliced green onions over the top for color and crunch. Serve hot.