Step 1: For the Mashed Potatoes: Place peeled and cubed potatoes in a large pot and cover with cold, salted water. Bring to a boil, then reduce heat and simmer until fork-tender, about 15-20 minutes. Drain well.
Step 2: Return potatoes to the hot pot over low heat for 1-2 minutes to evaporate any excess water. Remove from heat. Add 1/2 cup milk and 4 tbsp butter. Mash until smooth and creamy. Season with salt and pepper to taste. Keep warm.
Step 3: For the Green Bean Casserole: Preheat oven to 375°F (190°C). In a large pot of boiling salted water, blanch green beans for 3-5 minutes until bright green and tender-crisp. Drain and immediately plunge into an ice bath to stop cooking. Drain again thoroughly.
Step 4: In a large bowl, combine the blanched green beans, cream of mushroom soup, and 1/2 cup milk. Stir until well combined. Season with salt and pepper if desired.
Step 5: Pour the green bean mixture into an 8x8 inch (or similar) baking dish. Top with 1/2 cup of the crispy fried onions.
Step 6: Bake for 20-25 minutes, or until bubbly. Remove from oven, sprinkle the remaining 1/2 cup crispy fried onions over the top, and return to the oven for another 5 minutes, or until the onions are golden brown. Serve hot alongside the mashed potatoes.