In a Dutch oven, heat the oil over medium heat until it shimmers. Add the onion and sauté until lightly golden brown, 6 to 7 minutes. Add the garlic and sauté until fragrant, 1 to 2 minutes.
Add the lamb, season with the salt and pepper, and cook, stirring occasionally, until browned. Add the rice and stir to combine. Stir in the cinnamon, nutmeg, and cumin.
Add the chicken stock and bring to a boil. Reduce the heat to low, cover, and cook, until all the liquid is absorbed, about 15 minutes. Stir in the peas and dates.
Remove from the heat and let the rice rest, covered, for 10 minutes.
Meanwhile, in a small skillet, melt the butter over medium-low heat. Add the pistachios and stir until golden and fragrant, about 2 minutes, being careful not to burn them. With a slotted spoon, transfer the pistachios to a plate.
Add the lemon juice and half the pistachios to the rice and fluff the rice with a fork. Spoon the rice into a serving bowl or onto plates and garnish with the remaining pistachios.
