Preheat oven to 425°F. Place potatoes in a large pot; cover with cold salted water. Bring to a boil, reduce heat, and simmer until tender, 15 to 18 minutes, adding peas during the last 3 minutes of cooking. Drain and return vegetables to the pot. Add lemon zest and juice and 2 tablespoons oil; mash. Season with salt and pepper.  Meanwhile, heat remaining tablespoon oil in a large skillet over medium-high heat. Season steak with salt and pepper. Cook to desired degree of doneness, 3 to 5 minutes per side for medium-rare. Transfer steak to cutting board and let rest 5 minutes before slicing; reserve skillet. Reduce heat to medium. Add shallots to reserved skillet; cook, stirring occasionally, until tender, 2 to 4 minutes. Add wine and 1/4 cup water. Cook, scraping up any browned bits with a wooden spoon, until liquid is reduced by half, 4 to 6 minutes; remove from heat. Add butter and swirl pan to melt; stir in juices from cutting board. Season with salt and pepper. Serve steak topped with sauce and potatoes alongside.