Creating this amazing Pepper Jack Potato Casserole is simple when you follow these detailed steps:

1. Preheat Oven: Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking dish with butter to prevent sticking.
2. Prepare Potatoes: Peel and slice the russet potatoes thinly. This will ensure they cook evenly in the casserole.
3. Sauté Onion and Garlic: In a skillet, sauté the diced onion in a little oil over medium heat until translucent, about 3-4 minutes. Add minced garlic and sauté for an additional minute until fragrant.
4. Combine Ingredients: In a large bowl, mix the sour cream, milk, sautéed onion and garlic, paprika, salt, black pepper, and cayenne pepper. Stir until well combined.
5. Layer Potatoes and Cheese: In the greased baking dish, layer half of the sliced potatoes. Pour half of the sour cream mixture over the potatoes. Sprinkle half of the pepper jack and cheddar cheeses on top. Repeat with the remaining potatoes, sauce, and cheeses to create another layer.
6. Cover with Foil: Cover the baking dish with aluminum foil to prevent direct heat from burning the cheese.
7. Bake: Place the casserole in the preheated oven and bake for 45-50 minutes. Check for doneness by piercing the potatoes with a fork; they should be tender.
8. Uncover and Brown: Remove the foil and bake for an additional 10-15 minutes, allowing the top layer to become golden and bubbly.
9. Cool Slightly: Once out of the oven, allow the casserole to cool for 5-10 minutes. This will help it set for easier serving.
10. Garnish and Serve: If desired, garnish with chopped green onions before serving for a touch of color.

These step-by-step instructions will guide you through creating the perfect Pepper Jack Potato Casserole effortlessly.