Halve the tomatoes crosswise and carefully scoop out and discard the pulp and seeds, careful not to puncture through the tomato. Sprinkle the inside with salt and pepper, set aside.
Preheat oven to 350F.
In a medium bowl, combine the artichoke hearts, spinach, and green onions. In the bowl of a food processor, add the cream cheese, butter, sour cream, thyme, salt to taste (1/2 tsp.), and Parmesan cheese. Whirl until smooth. Fold this cream cheese mixture into the spinach mixture until just combined.
The stuffed tomatoes can be completely made ahead of time and then baked when ready to serve.
Mix together the panko crumbs and butter in a small bowl. Set aside.
Spray a 9x13" pan with cooking spray. Stuff tomatoes with the cream cheese spinach mixture and top with the panko crumbs. If making ahead, you can leave the panko crumbs off until ready to bake. Bake for 10 min. or 5 min., longer if they are cold from the fridge. Do not overcook.