Heat 2 tablespoons olive oil in a large skillet over medium heat. Add the raisins and pistachios and cook, stirring occasionally, until the raisins swell up and the pistachios toast slightly, about 1 minute. Remove raisins and pistachios to a small plate and set aside for now.
Using medium heat again, add the remaining 2 tablespoons olive oil. Add the cumin seeds and let sizzle about 10 seconds, then add the onion and bell pepper and cook until softened and starting to brown, about 5 minutes. Add the Serrano pepper and cook 2 more minutes, adding the teaspoon salt, 1/2 teaspoon of brown sugar, and 1/4 teaspoon of black pepper. Add the meat, breaking up the big lumps, and cook about 5 minutes, stirring occasionally.
Add the prepared sauce to the meat and onion mixture in the skillet. Stir and bring to a boil, then reduce the heat to low. Cook uncovered until the sauce reduces and thickens, about 12-15 minutes.
You may serve the Sloppy Joes at this point, but after the sauce has thickened, I like to add about 3 tablespoons of water back in (just a good splash), then put a lid on the Sloppy Joe mixture, leave on low heat with the Serrano pepper still in the mix, and let cook another 10-20 minutes longer. This only enhances and deepens all the flavors.
Remove the Serrano pepper. Stir in the raisins and pistachios. Taste for seasoning. Stir in the cilantro right before serving.
Toast buns or rye bread and fill with the Sloppy Joe mixture.