Add the cumin, pumpkin seeds, serrano chiles, cilantro, scallions, garlic, tahini, the juice of 2 limes, in a blender. Add about 3/4 cup of water and blend. The sauce should thick, very smooth but pourable. If it is too thick, add more water about a couple tablespoons at a time until you get the right consistency. Add two large pinches of salt. Taste as you go and add more salt if it needs it. Remember this is the sauce for the tacos so it should be flavorful (and sufficiently salted).