In a large skillet, drizzle the olive oil over medium heat.
Add the onion and cook, stirring occasionally, until beginning to soften.
Stir in the garlic and cook until just fragrant, 30 seconds-1 minute. Stir in the kale and cook just until tender and vibrant green. Remove from heat and allow to cool. Squeeze out any excess moisture.
In a large bowl, combine the ricotta, 3/4 of the mozzarella, 3/4 of the Parmesan, egg, parsley, basil, salt, and pepper.
Stir in the cooled kale mixture.
Preheat oven to 350˚F (180˚C).
Bring a large pot of salted water to a boil.
Once boiling, add the shells and cook until barely tender, al dente. Gently drain and allow to cool just enough to handle.
In a 9x13 inch (23 x 33 centimeter) baking dish, spread about 1 cup (240 milliliters) of the tomato sauce to cover the bottom.
Fill each shell with about 2 tablespoons of the kale mixture.
Place the filled shells in a single layer in the prepared baking dish.
Spoon the remaining tomato sauce over the shells. Sprinkle with the remaining mozzarella and Parmesan.
Cover the baking dish with foil and bake in preheated oven for 25 minutes.
Remove the foil and bake for an addition 10-15 minutes, until golden and bubbly.
Serve warm.