High-heat cooking oil (such as sunflower, safflower or grape seed oil), as needed
8 large portobello mushrooms, stemmed and gills removed (TIP: Use a small spoon to remove dark brown gills from mushrooms.)
1/3 cup grated reduced-fat Swiss cheese
Mist a large nonstick skillet with cooking spray and heat on medium-high. Add turkey and cook, breaking up with a wooden spoon, until browned and cooked through, about 5 minutes. Stir in scallions, tomatoes, goat cheese, panko and pepper; heat through for about 1 minute, stirring frequently, until combined and cheese has melted. Remove from heat.
Heat grill to medium-high and lightly oil grate. Mist mushrooms all over with cooking spray. Add to grill stemmed side down; close lid and grill until juices start to release, about 4 minutes. Turn over; close lid and grill until tender, about 2 minutes. Remove from heat; fill each mushroom with turkey mixture and sprinkle with Swiss cheese, dividing evenly. Reduce heat to medium-low and return mushrooms to grill stuffed side up; close lid and cook until stuffing is warmed through and Swiss cheese is melted and bubbly, about 3 minutes.