You can vary the filling by substituting robust mustard greens or collards, or milder greens, such as spinach, escarole or Swiss chard. (Cook the greens until just tender in Step 2.)
- To prepare peppers: Preheat oven to 400°F. Halve peppers lengthwise through the stems, leaving them attached. Remove the seeds. Lightly brush the peppers outside and inside with oil; sprinkle the insides with salt and pepper. Place, cut-side down, in a 9-by-13-inch baking dish. Bake until peppers are just tender, 10 to 15 minutes. Let cool slightly. Turn cut-side up.
- To prepare filling: 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.