Heat the oven to 200°F.

• Remove 12 tortillas from the package; save the rest for another use.

Lay a damp kitchen towel on a sheet pan. Arrange the 12 tortillas on one end of the towel, overlapping them slightly; cover them with the other end of the towel. Heat in the oven until warmed through and just pliable, 10 to 12 minutes. Remove from the oven and let stand, still covered, until ready to use.
While the tortillas warm, prepare the scallions.

• Trim the root ends from the scallions; cut the scallions in half lengthwise, then crosswise into 2-inch lengths.

In a large frying pan over medium-high heat, warm 1 to 2 tablespoons oil until hot but not smoking. Add the scallions and cook, stirring occasionally, until lightly charred, 4 to 6 minutes. Using a slotted spoon or tongs, transfer to a plate. Do not clean the pan.
While the scallions cook, prepare the corn and black beans.

• Shuck the corn. Lay the cobs flat and cut the kernels from the cobs.
• Rinse the black beans.
• If using, remove the stem, ribs, and seeds from the jalapeño; finely chop the jalapeño. Wash your hands after handling.

In the same pan used for the scallions, if dry, add 1 teaspoon oil and warm over medium-high heat until hot but not smoking. Add the corn, beans, mole poblano spice blend, ¼ cup water, and as much jalapeño as you like and season with salt and pepper. Cook, stirring occasionally, until the corn is just tender, 3 to 4 minutes. Using a slotted spoon, transfer the corn–black bean mixture to a medium bowl.
While the corn and beans cook, prepare the garnishes.

• Cut the limes into wedges.
• Crumble the queso fresco.
• Cut the avocados in half. Remove the pits, scoop out the flesh, and cut the flesh into ½-inch pieces.
• Coarsely chop the cilantro.

Set out the tortillas, corn–black bean mixture, scallions, lime wedges, queso fresco, avocados, cilantro, and pumpkin seeds and invite everyone to assemble their own tacos.