Preheat the oven to 375°. Spray a small casserole dish with vegetable spray and set aside.
In a small saucepan, melt two tablespoons of butter over medium high heat. Sprinkle the flour, 1/4 teaspoon kosher salt and the white pepper into the melted butter and whisk until well combined and bubbly. Gradually add the milk stirring constantly and scraping the sides. Heat until the mixture begins to bubble and cook for one minute, stirring constantly until it thickens. Remove from heat stir in 1 cup of the cheese until its melty and gooey. Spoon 2-3 tablespoons of the cheese sauce into the bottom of the casserole dish. Set aside.
In a medium pan with a lid, add the water to the pan and heat over medium high heat until water almost boils. Stir in the kale, sprinkle with 1/4 teaspoon salt, cover with lid and reduce heat to medium-medium low, cooking just until the kale wilts and water has mostly evaporated -- about 2-3 minutes. Transfer the kale to the casserole dish, spreading it evenly over the bottom.
Remove the watery seeds from the tomatoes and arrange cut side up in the casserole dish. Sprinkle with 1/4 teaspoon salt.
Whisk the eggs and 1/4 teaspoon salt in a small bowl. Melt 2 tablespoons butter over medium low heat in the pan that you cooked the kale in and add the eggs. Continue to whisk the eggs until you have a very soft scramble, stirring constantly. (Eggs should be very soft and a little wet).
Evenly divide the scrambled eggs over the tomatoes. Spoon the cheese sauce over the gratin (note: sauce will be very thick) and sprinkle with remaining cheese.
Bake for 20 minutes, then turn the broiler onto high and cook for 3-4 minutes more until the top begins to brown. Let the gratin rest for 5 minutes before serving.