Instructions

• In a small bowl whisk together the marinade ingredients: 1 clove garlic (minced), 1/2 tablespoon brown sugar, 1/3 cup soy sauce, 1/3 cup water, 1 tablespoon balsamic vinegar, 2 teaspoons corn starch, and 1 1/2 tablespoons sriracha hot sauce.
• Pour the marinade into a large zip-top bag, add the 1 lb. thinly sliced flank steak (cut diagonally across the grain into thin strips), seal the bag, and toss to coat all pieces evenly. Refrigerate for at least 10 minutes.
• Heat 1 1/2 tablespoons sesame oil in a large skillet over medium-high heat.
• Add the peppers (red, orange, yellow, thinly sliced), the 1 medium red onion (thinly sliced), 2 tablespoons grated fresh ginger, and the 6 garlic cloves (minced) to the skillet. Sauté, stirring occasionally, until the vegetables soften, about 5 minutes. Transfer the vegetables to a clean plate.
• Add the 1 tablespoon unsalted butter to the hot skillet. Using a slotted spoon, remove about half the steak from the bag (reserve the remaining marinade in the bag) and add the steak to the skillet in a single layer. Cook, stirring occasionally, until the steak is nicely browned, about 4 minutes. Transfer the cooked steak to the plate with the vegetables.
• Repeat: add more butter to the skillet only if needed, then add the remaining steak (removed with the slotted spoon so the reserved marinade stays in the bag) and cook until browned, about 4 minutes. Transfer this cooked steak to the plate with the other steak and vegetables.
• Add the remaining 1 tablespoon sesame oil to the skillet. Pour the reserved marinade from the bag into the skillet, bring to a simmer, and cook, stirring, until the sauce thickens, about 1 minute.
• Return the cooked steak and vegetables to the skillet, add the 2 handfuls of fresh baby spinach, and stir to combine. Cook 1–2 minutes more, until the spinach is wilted and everything is heated through. Remove from heat.
• Serve immediately with rice, quinoa, or couscous.