These tacos are a favorite of chef Rick Bayless, who first tasted them at a market stall in Toluca, Mexico, where they're considered to be good, old-fashioned peasant food.
Warm the tortillas: Place a vegetable steamer in a large saucepan filled with 1/2 inch of water. Bring to a boil. Wrap tortillas in a heavy kitchen towel, lay them in the steamer, and cover with a tight lid. Boil 1 minute, turn off the heat, and let stand without opening for about 15 minutes.
Prepare the filling: While tortillas are steaming, bring 3 quarts salted water to a boil in a large pot. Add greens, and cook until barely tender, 1 to 2 minutes. Drain in a colander, and spread out on a large plate or baking sheet to cool. When cool enough to handle, roughly chop.
In a large skillet, heat oil over medium-high heat. Add onion, and cook, stirring frequently, until golden brown, about 10 minutes. Add garlic, stir for 1 minute, then add greens, and stir for about 1 minute, just long enough to heat them through. Season with salt.
Prepare the tacos: Scoop the filling into a deep, warm serving dish, and sprinkle with cheese. Serve with the warm tortillas, salsa, and cilantro sprigs.