Makes 4 servings. Difficulty
1-1/4 pounds boneless sirloin steak, about 1" thick Salt and freshly ground pepper 1 small bunch of spinach, tough stems discarded 1-2 Tablespoons balsamic vinegar 2 Tablespoons olive oil 1/4 cup fresh basil leaves, finely shredded 3 Tablespoons fresh sage leaves, finely shredded 1 Tablespoon unsalted butter
Cut sirloin against the grain into 12 even slices. Pound steak slices between plastic wrap to a 1/4" thickness. Generously season meat on both sides with salt and pepper.
Mound spinach on 4 large plates and drizzle with balsamic vinegar.
In a large heavy skillet, heat 1 tablespoon olive oil until almost smoking. Add half the steak slices and cook over high heat until just browned, about 1-1/2 minutes. Turn meat, sprinkle with half the basil and sage; cook until browned but still very rare, about 1 more minute.
Transfer steak slices to a plate; repeating process with remaining olive oil, steak and herbs.
Add butter to skillet and melt, scraping up any browned bits.
Return steak, herbs, and accumulated juices to pan and warm over moderate heat, turning slices once, for 1 minute.
Arrange steak slices on spinach, spoon pan juices over meat and serve.