Preheat grill to medium-high heat. Lightly brush with oil to prevent sticking.
In large bowl, add beef and seasoning. Using your hands, gently mix to combine. Form into four patties.
Grill patties, with lid closed, 4–5 min per side, or until cooked through.
Meanwhile, slice tomato and onion. Slice buns open; add to grill, face down, for final 1 min of grilling patties. At this time, add 1 slice cheese on top of each patty to melt.
Tuck burgers into buns. Top with tomato, onion, spinach, and additional toppings, if desired.