Cut the pumpkin into manageable chunks for grating, if needed, then finely grate it on a box grater (or using a food processor with the grating disk) until you have 1 heaping cup. Dice the remaining pumpkin into ½-inch pieces. Set both aside.
Rinse the rice until the water runs clear. Drain and set aside.
Melt the butter in a large lidded pot or Dutch oven over medium-high. Add the leek and half the sliced celery, and season with salt and pepper. Cook for 4 to 5 minutes, stirring occasionally, until softened and golden brown in parts. 
Add the grated pumpkin, drained rice and turmeric. Toast the rice for 1 to 2 minutes, stirring frequently, until the grains are coated. Stir in the diced pumpkin, 1 ½ cups of water (or the suggested amount, if the rice package directions differ) and season with salt and pepper until the cooking liquid tastes like a well-seasoned broth. 
Bring the mixture to a simmer over medium-high, add the thyme, cover and cook over medium-low for 15 minutes. Remove from heat and keep covered for an additional 10 minutes, until all liquid is absorbed.
Fluff the rice with a fork and remove the thyme sprigs. Stir in the peanuts, scallions, remaining sliced celery and reserved celery leaves.