In a shallow dish, whisk together the soy sauce, balsamic vinegar, brown sugar, and garlic. Add the steak, turning to coat. Cover and let rest 15 minutes. (You can also refrigerate the steaks in the marinade for up to 8 hours.)

Grill over medium-high heat for 4-5 minutes on each side, or until cooked to our desired level of doneness. Transfer to a cutting board; cover loosely with foil and let rest 10 minutes. Slice thinly, across the grain.

While the steak rests, heat the olive oil in a large skillet. Add the onions and cook, stirring frequently, until soft, about 5 minutes. Add the spinach and cook one minute, or until just wilted. Transfer to a serving bowl; add the strawberries, blue cheese, and dressing.

Slice the ciabatta into 6 sections; split each piece crosswise. Assemble your sandwiches by piling sliced steak on a piece of bread then topping with the spinach salad.