“`html

1. Start by washing and peeling the russet potatoes. Cut the potatoes into even-sized cubes, about 1 to 2 inches.
2. Place the cubed potatoes in a large pot and cover them with cold water. Add a generous pinch of salt to the water.
3. Bring the pot to a boil over medium-high heat. Once boiling, reduce the heat to medium and let the potatoes simmer for about 15-20 minutes, or until they are fork-tender.
4. Once cooked, drain the potatoes in a colander and let them sit for a minute to allow excess moisture to evaporate.
5. In a large mixing bowl, add the drained potatoes. Using a potato masher or a hand mixer, begin mashing the potatoes until they reach your desired level of creaminess.
6. Add the softened butter to the mashed potatoes and mix until incorporated.
7. Mix in the sour cream and milk, adjusting the amount of milk based on how creamy you want your mixture to be.
8. Stir in the garlic powder, onion powder, and season with salt and pepper to taste.
9. Fold in the shredded cheddar cheese and grated Parmesan cheese.
10. Preheat your oven to 350°F (175°C).
11. Grease a 9×13-inch baking dish with a little butter or cooking spray.
12. Transfer the mashed potato mixture into the prepared baking dish, spreading it out evenly with a spatula.
13. If desired, sprinkle the breadcrumbs evenly over the top of the mashed potatoes.
14. For an extra cheesy touch, sprinkle a little more shredded cheddar cheese on top of the breadcrumbs.
15. Place the casserole in the preheated oven and bake for about 25-30 minutes, or until the top is golden brown and the edges are bubbling.
16. Remove the casserole from the oven and let it sit for about 5-10 minutes before serving.
17. Before serving, sprinkle the chopped green onions over the top of the casserole.
18. Serve the cheesy mashed potato casserole warm.

“`