Preheat the oven to 350°F. In a saucepan over medium heat, melt the butter. Add the flour and stir or whisk to blend. Add the milk gradually, stirring or whisking constantly until the sauce thickens. Remove the pan from the heat, add the egg yolks, and beat to blend. Add the cheese and mix until combined. Add the salt and pepper and mix, then set aside. Slice the tops off of the tomatoes and scoop out the pulp, leaving a ½-inch-thick shell wall. Invert the tomato shells onto paper towels and set aside to drain. In a large, chilled bowl, beat the egg whites until stiff but not dry. Gently fold the whites into the cheese mixture. Set the tomato shells into six small soufflé dishes or custard cups or side-by-side in a 9x9-inch (or similar) baking dish. Spoon the cheese soufflé mixture into the tomatoes. Bake for 35 to 40 minutes, or until puffed and light brown. Garnish with chopped chives.