Make the marinade by placing the mustard, balsamic vinegar, olive oil, garlic, and basil in a large shallow dish and whisking to combine. Add salt and pepper to your taste.

Season the steaks with black pepper and add to the marinade. Turn the steak with tongs a few times to evenly coat. Marinate the steaks for at least 30 minutes at room temperature or up to 10 hours in the refrigerator.

Heat an outdoor grill to medium heat, or a grill pan on a stovetop on medium-high.

Remove the steak from the marinade, allowing the excess marinade to drip off. Lightly season with salt and pepper on the first side, if desired, and grill steak for 4-5 minutes. Turn steak, season again to taste, and cook for a further 3-5 minutes or until the steak reaches your desired level of doneness.

Transfer cooked steaks to a platter and let rest for 10 minutes before slicing the diagonal.

Meanwhile, place the baby spinach, red onion, cherry tomatoes, and pepper into a large bowl and gently toss to combine. Add the lemon juice and extra-virgin olive oil to the bowl, and season with salt, and black pepper to taste then toss again.

Transfer the salad to serving plates and top with sliced steak and crumbled feta. Enjoy!