Slice beef horizontally almost all the way through; spread open butterfly fashion. Place beef between 2 sheets of wax paper; with meat mallet or bottom of heavy saucepan, gently pound until very thin. Remove and discard wax paper; set beef aside. In large nonstick skillet, heat 2 Tbsps water; add spinach. Cook over medium-high heat, stirring occasionally, until wilted. Drain, discarding liquid; set spinach aside to cool. Sprinkle beef on both sides with salt and pepper; place onto work surface. Sprinkle beef evenly with cheese; top with cooled spinach. Starting from shortest end, roll beef to enclose filling. Place medium cast-iron skillet over medium-high heat; add beef roll, seam-side down. Cook, turning as needed, 1 minute, until browned on all sides. Transfer skillet to oven; bake 15 minutes, until cooked through. Remove from oven; let stand 5 minutes. Slice beef roll crosswise into 12 equal pieces; arrange on serving platter.
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