In a cast iron or other heavy-bottomed skillet, toast the pumpkin seeds over medium heat until golden, 6 to 8 minutes. Transfer to a food processor. In the same skillet, cook the tomatillos, garlic, and jalapeño over high heat, turning occasionally, until charred in spots, 5 to 10 minutes. Transfer to the food processor with the pumpkin seeds.
Add the cilantro, lime juice, and ground cumin to the food processor and pulse everything until combined (mixture will still be coarse). Season to taste with salt and pepper and pulse again to combine. Store the sauce in the refrigerator for up to one week.