1: Set up your grill for direct grilling and build a hot fire. Brush and oil the grill grate.

2: Make the Pipian Sauce: Grill the tomatillos, poblano, serranos, onion, garlic, scallions, and lettuce leaves until golden brown, 2 to 4 minutes per side. Transfer to a cutting board to cool to room temperature, then cut into 1-inch pieces. (For a milder salsa, seed the chiles.)

3: Toast the pumpkin seeds in a dry cast iron skillet (not nonstick) over medium heat until they begin to brown and pop, 3 minutes. Shake the pan as the seeds cook; do not let them burn. Transfer the seeds to a shallow bowl to cool.

4: Set 3 tablespoons of seeds aside for a garnish, then grind the remaining seeds to a fine powder in a food processor, running the machine in short bursts. Work in 1/2 cup of water or clam broth.

5: Place the grilled vegetables in the food processor and puree. Work in the cilantro, parsley, cumin, and lime juice. Work in an additional 1 cup water or clam broth, adding more as needed to obtain a thick but pourable sauce.

6: Heat the olive oil in a large, deep skillet over medium heat. Add the pumpkin seed mixture and cook until dark, thick, and fragrant, about 5 minutes, stirring frequently to prevent splattering or scorching.

7: Stir in the tomatillo mixture and continue cooking the sauce until thick and richly flavored, 15 to 20 minutes, stirring often. During the last 5 minutes of cooking, stir in the epazote (if using). Remove the sauce from the heat and taste for seasoning, adding salt to taste and/or more lime juice; the sauce should be highly seasoned. Set the sauce aside and keep it warm.