Preheat griddle or skillet. In a small bowl, combine cream cheese, artichoke hearts, frozen spinach, garlic, Italian seasoning and shredded Jack cheese in a bowl. Set aside.
Season ground beef with desired amount of salt and pepper and form into 4 patties. Grill burgers and onions on medium heat for 4-5 minutes per side. Turn onions as they brown. When burgers have browned, move to the cooler side of the griddle pan, or to a warmed plate.
Butter bread slices and arrange on the griddle, cooking 1-2 minutes per side, constantly turning to brown up both sides.
When toasted, arrange hamburger patties on each of the four slices, topping with a generous amount of spinach mixture, then top with second slices of bread.
Cover and continue to toast for about 2 minutes, turning and toasting the second side for about 2 minutes more.