Set oven to BROIL and position one oven rack 8-10 inches from the top.
PREPARE TOMATOES: Line a small baking pan with foil and spray with oil. (Alternatively, spray a pyrex baking dish, about 9 inches, with oil.) Add halved tomatoes, Dijon, green onions and garlic. Mix to combine. Sprinkle gruyere cheese evenly over the top.
PREPARE CRUMB TOPPING: Add all crumb topping ingredients to a small bowl and mix well to combine. If not using Parmesan cheese, add salt to taste. Sprinkle crumb topping over tomatoes and cheese dish evenly. At this point, you can set the dish aside to be broiled before serving.
BROIL: Broil tomato dish 8-10 inches from the heat source (top) for about 3 minutes or until cheese is bubbling and crumb topping is lightly browned. Watch that it doesn't burn.
SERVE: Serve directly from dish (if you used a pyrex dish) or, if using a pan, transfer to a shallow serving dish with a spatula. (I line my dish with fresh baby spinach or kale, then spoon tomatoes on top). Serve immediately.