In a small saucepan, combine 2/3 cup water, the sugar and 4 teaspoons salt. Set over high heat. Swirl the pan until the sugar is dissolved, then remove from the heat and stir in the vinegar. When cooled to room temperature, add the tomatillos, transfer to a resealable 1-quart plastic bag, press out the air, seal and refrigerate for 6 hours or, better, overnight.
To make the dressing, pour the lime juice and 2 tablespoons water into a blender. Add the pumpkin seeds and chile and blend until smooth; then, with the motor running, drizzle in the oil. (If you happen to have it, this is a great place to add a teaspoon or so of dark pumpkin seed oil.) Taste and season with salt, usually about ½ teaspoon.
Drain the tomatillos. In a large bowl, toss the lettuce with 2 to 3 tablespoons of the dressing and the cheese, coating all the leaves evenly. (Refrigerate the remaining dressing, covered, for another salad; it will last for several weeks.) Divide between four salad plates, sprinkle with the pickled tomatillos and a few pumpkin seeds and serve right away.
Pickled Tomatillo Salad as a passed appetizer: When you want something delicious and light to pass around at a party or the beginning of a nice dinner, choose small leaves inner leaves of little gem or Boston lettuce and line them up on a platter. Slice more leaves crosswise a little wider than ¼ inch. Toss with dressing and cheese, then divide the sliced lettuce “salad” among the leaves on the platter. Top each with small pieces of pickled tomatillos, a little more cheese and a few pumpkinseeds. This is one of my favorite bites to serve as a snack with drinks.