Bring 2 cups salted water to a boil in a large wide pan.
Stir in kale, cover and cook until tender, 10 to 12 minutes. Drain, rinse under cold water; squeeze dry. Finely chop.
Heat oil in a large nonstick skillet over medium heat.
Add onion and chopped bell pepper; cook, stirring often, until onion is golden, 6 to 8 minutes.
Add garlic and cook, stirring, for 30 seconds.
Stir in the kale. Remove from the heat and let cool slightly.
Stir in rice, Parmesan, 2 tablespoons pine nuts and lemon juice. Season with salt and pepper. Divide the filling among the pepper halves. Sprinkle with the remaining 2 tablespoons pine nuts.
Add 2 tablespoons water to the baking dish.
Cover the peppers with foil and bake until heated through, 15 to 20 minutes. Uncover and bake for 5 minutes more. Serve hot.