Preheat the oven to 350°F (175°C).

Peel russet potatoes and dice into 1/2-inch cubes; place in a bowl of cold water to prevent browning.

Cook chopped bacon in a large skillet over medium heat for 8–10 minutes until crispy. Transfer bacon to paper-towel-lined plate and reserve 2 tablespoons bacon fat in the skillet.

Add butter to the skillet; once melted, whisk in flour to form a roux. Cook, whisking constantly, for 2–3 minutes until lightly golden.

Gradually pour in whole milk while whisking. Continue cooking 5–7 minutes until the sauce thickens enough to coat the back of a spoon.

Remove skillet from heat; stir in shredded cheddar and gruyere until melted and smooth. Season with salt, black pepper, garlic powder, and cayenne pepper.

Drain potatoes and add them to the cheese sauce along with the cooked bacon. Gently fold until all pieces are evenly coated.

Spread the mixture into a 9x13-inch baking dish.

Sprinkle breadcrumbs evenly over the top of the casserole.

Bake for 40–45 minutes, until the top is golden brown and the edges are bubbling.

Remove from oven and let rest 5 minutes. Garnish with sliced green onions and serve hot.