Stir together first 6 ingredients in a large bowl until well blended; stir in cheeses.
Head griddle or nonstick skillet over medium-high heat. Spread 1/3 c cheese mixture on each of 4 slices. Top with tomato slices and remaining 4 bread slices. Sp[read one side of sandwiches with 1tbsp butter.
Coat hot griddle or skillet with cooking spray. Place sandwiches, buttered sides down, on griddle or in a skillet. Spread tops with remaining butter. Cook two to three minutes on each side or until golden brown. Serve immediately.