Garlicky tender greens, soft, fresh cheese, and seared onions - turn meatless tacos into savory fare for everyone.
Warming the tortillas: Set up a steamer (a vegetable steamer in a large saucepan filled with 1/2 inch of water works well); heat to a boil. Wrap the tortillas in a heavy kitchen towel, lay in the steamer and cover with a tight lid. Boil 1 minute, turn off the heat and let stand without opening the steamer for about 15 minutes.
The greens-and-onion filling: While the tortillas are steaming, prepare the filling. Bring 3 quarts of salted water to a boil in a large pot. Add the greens and cook until barely tender, about 2 to 3 minutes for the lamb’s quarters, 1 to 2 minutes for the chard. Pour into a colander, then spread out on a large plate or baking sheet to cool. When cool enough to handle, roughly chop.
Heat the oil in a large (10- to 12-inch) skillet over medium-high. Add the onion and cook, stirring frequently, until golden brown, about 10 minutes. Add the garlic, stir for 1 minute, then add the greens and stir for a minute or so longer, just enough to heat them through. Taste and season with salt.
The tacos: Scoop the mixture into a deep, warm serving dish, sprinkle with the cheese if you’re using it, and carry to the table along with the warm tortillas in a cloth-lined basket for each of your guests to assemble tacos al gusto. For a sit-down appetizer, you may want to roll the tortillas around the filling, arranging them by twos on each plate, then spoon the salsa in a strip across their middles. Sprinkle with cheese and top with sprigs of cilantro.