To start the sauce, put the walnuts and cashews in a medium bowl. Cover with boiling water and set aside to soak while you prepare the peppers.
Roast the poblano peppers on an open flame for 1 to 2 minutes, until charred on all sides. (Alternatively, heat the oven to the highest broiler setting, put the peppers on a rimmed baking sheet lined with aluminum foil, and broil on the middle rack for 1 to 2 minutes on each side, until charred.) Transfer the peppers to a bowl, cover with plastic wrap, and let sit for 10 minutes to release steam.
Meanwhile, heat 1 tablespoon oil in a large skillet or sauté pan over medium heat. Add one-third of the mushrooms and cook, undisturbed, for 3 minutes, or until golden. Flip the mushrooms, lightly sprinkle with salt, and cook for an additional 3 minutes. Transfer the mushrooms to a bowl. Cook the remaining mushrooms in the same way, adding another
1 tablespoon oil to the pan for each batch and transferring the mushrooms to the bowl as they are done.
Heat the remaining 2 tablespoons oil in the same pan over medium heat. Add the plantain and cook for 2 minutes on each side, or until golden. Transfer to the bowl with the mushrooms. Add the onion to the pan and sauté for 3 minutes. Add the garlic and sauté for 1 minute. Add the tomatoes and sauté for 5 minutes.
Add the apple and pear and sauté everything together for 7 minutes. Add the cooked lentils, broth, and raisins and simmer for 10 minutes, or until the liquid is mostly absorbed. Return the mushrooms and plantain to the pan, along with the oregano and cumin, and season with salt to taste. Turn the heat down to low.
Remove the charred skin from the poblano peppers. Cut a slit down the side of each pepper and remove the seeds. Spoon a generous amount of the mushroom filling inside each, then press the peppers closed.
To finish the sauce, drain the cashews and walnuts and rinse under cool running water. Combine the nuts, milk, sugar, and salt in a blender and blend on high for 1 minute, or until smooth.
To serve, place a stuffed pepper on each plate, cover with the creamy sauce, and top with pomegranate seeds and chopped cilantro.