Mix 2 tablespoons olive oil, balsamic vinegar, 1 tablespoon kosher salt, 1 tablespoon black pepper, garlic, sazon seasoning, and red pepper flakes together in a large bowl. Add flank steak; mix until well-coated.
Heat a large skillet over medium-high heat. Add steak in a single layer; cook until browned, about 3 minutes per side.
Whisk remaining 2/3 cup olive oil, juice of 2 limes, zest of 1 lime, remaining 1 tablespoon kosher salt, remaining 1 tablespoon black pepper, and garlic powder in a large bowl to make dressing.
Place onion, baby spinach, tomatoes, mozzarella cheese, and basil in bowl with dressing; toss until combined. Serve salad with steak.