Defrost pie shells. Then, lightly paint egg wash on the edges. [This will make it prettier.]
Bake pie shells at 350° for half of their directed baking time; about 7 min. You want it to be firm and lightly golden, not "done." Let it cool, and do not turn off oven.
While pie shell bakes, thinly slice fresh tomatoes and lay flat on a double-layer of paper towels. Sprinkle with salt and cover with another single layer of paper towel. [This draws out excess moisture, so your pie isn't soggy.]
Dice and sauté sweet onion in butter; set aside.
Next, thoroughly combine cheese mixture ingredients.
When pie shell is cool, smooth out a thin layer of cheese mixture into the bottom of the shell.
Place a good, even layer of tomato slices and lightly sprinkle with Italian seasoning. Top layer of sliced tomatoes with ½ of sautéed onion. Repeat this once more, and leave about a ½ inch of space near the top of the crust.
Spread and smooth out remaining cheese mixture on top; It is fine to have it slightly bulging, but do not smooth out onto the crust.
Bake for 30-35 minutes, until bubbly. Be sure to let it stop bubbling before serving.