Preheat oven to 350 degrees. Spread almonds on a rimmed baking sheet. Toast in oven, tossing occasionally, until golden, 5 to 8 minutes. Transfer to a small bowl; set aside.
Heat a large skillet over medium-high. Season steaks with salt and pepper; cook in skillet, turning once, until medium-rare, 3 to 5 minutes per side. Transfer to a plate, and tent loosely with aluminum foil (reserve skillet).
Place spinach and grapes in skillet; toss with pan juices until spinach is slightly wilted, 1 to 2 minutes. Stir in vinegar and toasted almonds. Serve steak with spinach.