1. Combine first 5 ingredients in a large bowl until blended. (Do not overwork meat mixture.) Shape mixture into 4 (5-inch) patties.
2. Grill, covered with grill lid, over medium-high heat (350° to 400°) 5 to 6 minutes on each side or until done.
3. Scoop out soft centers from bottom half of rolls, leaving 1/4-inch-thick shells. Place burgers in shells; top evenly with spinach and tomato slices, and cover with roll tops.