In a large skillet, over medium heat, melt butter. Stir in the onions, cooking until tender and translucent, about 3-5 minutes.
Immediately remove the skillet from heat. Mix in the sour cream, mayo, cheese, oregano, garlic stirring until equally incorporated and the cheese has melted. Salt and pepper the sauce, to taste.
Spray a 9x13 baking dish with non-stick cooking spray. Spread the 8 biscuits out evenly into the pan. Use your fingers to press them out further, until they're all touching and completely spread out over the bottom of the pan. Seal the seems together so the pan's covered and press the outer edges up the sides slightly, just enough to form a tiny lip.
Layer the sliced tomatoes evenly out over the thin layer of dough. Use a spatula to spread the cheese sauce evenly out overtop of the tomatoes. Sprinkle the prepared casserole with a little smoked paprika.
Bake the dish at 400 degrees for 25-27 minutes, or until the dough is cooked through and the cheese has bubbled and turned golden.
Remove the casserole from the oven and let the dish rest for 10 minutes before slicing into squares and serving.