Preheat oven to 425 degrees. Meanwhile, heat a large skillet over medium-high heat and add olive oil. Add onion and salt then cook approximately 3 minutes or until onion starts to turn translucent.
Add garlic and cook an additional 30 seconds. Add baby spinach and cook about 1–2 minutes, stirring frequently, just until spinach is dark green and wilted. Remove from heat and set aside to cool slightly.
Trim any tough tissue and excess fat from both sides of flank steak then butterfly the meat along the long side. Trim any edges or fold any flaps of meat back over itself so that you are left with a mostly rectangular steak. Pound with a meat tenderizer or meat mallet to even out the thickness of the steak.
Season surface of flank steak with salt and pepper and lay cooked spinach in an even layer across top of steak, leaving a 1½" border all the way around. Sprinkle breadcrumbs over spinach then distribute the feta and jack cheese evenly over the spinach.
Roll the flank steak up like a jellyroll. The grain of the meat should run the length of the cylinder after the meat has been rolled. Tie the meat at each end and approximately every 3" with kitchen twine to hold the roll securely closed.
Place roll on a baking sheet and brush or spray with oil. Season on all sides with salt and pepper then roast 40-50 minutes or until the internal temperature reaches 145 degrees directly in the center of the roll. Remove from the oven and allow it to rest 10 minutes before removing the kitchen twine and slicing.