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1
Line a 10-inch heavy skillet (preferably cast-iron; not nonstick) with foil, then heat over moderately high heat until hot but not smoking. Add 2 whole garlic cloves and half of tomatillos, cut sides down, and roast on top of stove until well browned, 4 to 5 minutes (skillet will begin to smoke at this point). Turn over garlic and tomatillos (tomatillos may stick to foil) and roast until well browned and tomatillos are completely soft, 3 to 4 minutes. Transfer roasted garlic and tomatillos to a food processor. Discard foil, then line skillet with a fresh sheet of foil and roast remaining 2 whole garlic cloves and remaining tomatillos in same manner, transferring as done to food processor. Cool garlic and tomatillos to room temperature in bowl of processor, about 30 minutes.
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2
Add chiles, half of cilantro, and 2 tablespoons water to mixture in processor, then pulse until it forms a chunky sauce, adding 1 tablespoon more water if too thick. Transfer to a bowl and stir in onion and 1 teaspoon salt.
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3
If using avocado leaves, toast, 1 at a time, in a dry 10-inch heavy skillet (without foil lining) over moderate heat, turning every few seconds, until lightly browned and aromatic, about 1 minute, transferring to a plate as toasted. Bring beans and remaining 2 cups water to a simmer in a 3-quart heavy saucepan, stirring occasionally, then add avocado leaves and simmer, uncovered, stirring occasionally, 20 minutes. Drain beans in a sieve set over a bowl, reserving liquid. Pure beans, 1/4 cup cooking liquid (reserve remainder), avocado leaves (if using), and chopped garlic in a food processor until smooth.
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4
Heat oil in a 10-inch heavy nonstick skillet over moderate heat until hot but not smoking, then add bean puree and cook, stirring, until very thick, about 5 minutes. Stir in remaining 3/4 teaspoon salt, then remove from heat and keep warm, covered.
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5
Halve and pit avocados, then scoop out flesh in large pieces and cut into 1/2-inch cubes.
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6
If necessary, stir some of reserved bean-cooking liquid in to make bean puree soft and easily spreadable. Divide warm beans among tostadas and spread evenly. Spoon about 1 tablespoon tomatillo sauce over each tostada, then divide avocado among tostadas and top with romaine, cheese, and remaining cilantro. Serve remaining tomatillo sauce on the side.
