Start by marinating the steak. In a large bowl, add the olive oil, balsamic vinegar, minced garlic, Dijon mustard, salt, and pepper and whisk until well combined. Add the flank toss to the bowl, toss to coat in the marinade, and allow to marinate for at least 20 minutes on the counter or covered for up to 12 hours in the fridge.
When you are about ready to cook the steak, prepare the couscous. Add the water and salt to a small pot, bring to a boil, then add the couscous and olive oil and stir to combine. Remove the pot from the heat, cover with a lid, and let stand for 5 minutes, then remove the lid, fluff the couscous with a fork, and set it aside.
Preheat a grill, grill pan, or cast-iron skillet to medium-high heat, then add the seasoned steak and cook for 4-5 minutes on one side, then flip and cook for an additional 4-5 minutes for medium-rare, 6-7 minutes for medium, or 8-9 minutes for medium-well, or until a thermometer inserted reaches 130–135°F for medium-rare, 140–145°F for medium, or 150–155°F of medium-well. Once cooked, transfer it to a cutting board to rest for at least 5 minutes, and then slice it – against the grain – into thin strips.
While the steak is resting, chop the lettuce, tomatoes, cucumber, red onion, and olives (optional), add them to a large bowl, drizzle with olive oil and lemon juice, and toss to combine. Taste and adjust seasoning with salt and pepper as needed.
Once the steak is rested, the couscous is cooked, and the veggies are ready, assemble the bowls. Dividing the ingredients evenly across 4 bowls, spread the hummus across the bottom of the bowl, top with couscous, mixed vegetables, slices of steak, and then sprinkle with crumbled feta cheese and parsley.
Any leftover steak and couscous can be cooled and stored in separate airtight containers for up to 4 days.