Prep ingredients: Preheat oven to 450°F with rack in the upper third. Peel potato, slice into 1⁄4-inch rounds. Halve shallot, peel and thinly slice. Finely chop 2 tablespoons shallot. Grate Parmesan. Combine cream cheese and 1 cup water in a medium skillet over medium-high. Whisk until cream cheese is melted, 1-2 minutes. Season with 1⁄4 teaspoon salt and a few grinds pepper.
Bake scalloped potatoes: Add potatoes and sliced shallots to the skillet, shaking to distribute into an even layer (potatoes won't be completely submerged), and bring to a boil. Reduce heat to medium, cover, and cook until slightly thickened, about 5 minutes. Top with Parmesan. Roast scalloped potatoes in upper third of oven until tender and browned on top, about 20 minutes.
Prep green beans & steaks: Trim stem ends from green beans. Pat steaks dry and, using a mallet or heavy skillet, pound to an even 3⁄4-inch thickness, if necessary. Season all over with salt and pepper. Heat 2 teaspoons oil in a medium skillet, preferably cast-iron, over medium-high until very hot.
Cook green beans: Add green beans, 1⁄4 teaspoon salt, and a few grinds pepper to hot skillet, and toss to coat. Add 2 tablespoons water, cover, and cook until crisp-tender, about 2 minutes. Uncover and continue to cook, stirring, until tender and charred in spots, about 3 minutes. Transfer to a bowl and cover to keep warm.
Cook steaks: Add 1 teaspoon oil and steaks to same skillet, and cook, turning once, until lightly charred, 3–4 minutes per side for medium-rare. Transfer steaks to a cutting board and let rest 5 minutes. Return skillet to medium heat, add 1 teaspoon oil and chopped shallots. Sauté until golden, about 30 seconds.
Cook sauce & serve: To shallots in skillet, add 2 tablespoons vinegar and 1⁄4 cup water; simmer over medium-high until thickened and glossy, about 2 minutes. Off heat, stir in 1 tablespoon butter and steak juices; season with salt and pepper. Thinly slice steaks across the grain. Serve steak with green beans and scalloped potatoes. Drizzle pan sauce over top.