Set a heavy ungreased skillet or griddle over medium heat. Lay the unpeeled garlic on the hot surface and let it roast to a sweet mellowness, turning occasionally, until soft when pressed between your fingers (you’ll notice it has blackened in a few small spots), about 15 minutes. Cool, then slip off the papery skins and roughly chop.
In a small saucepan, heat the oil over medium heat. Add the capers and fry until starting to brown and crisp slightly, 3 minutes. Add the ancho chile pieces and 2/3 of the chopped garlic cloves, cook until oil mixture is deeply aromatic and darker in color, another 2 to 3 minutes. Remove from heat and add balsamic vinegar and salt (usually ½ teaspoon). Allow to cool.
Cut around each avocado from stem to blossom end and back up again, then twist the halves apart. Dislodge the pit and scoop the avocado flesh into a large bowl. Add the lime juice, black pepper, remaining roasted garlic and Mexican oregano. Coarsely mash everything together. Taste and season with salt, usually about ¾ teaspoon.
To assemble the tostadas, refresh the tostadas by brushing them in oil and flashing them in a 350 degree oven for a few minutes— you can skip this step, but I think the tostadas taste much better if they’re been refreshed.
Directly over a gas flame, toast the whole chard leaves until blackened in spots and aromatic. Set aside to cool, roughly chop.
Slice the fish into thin slices, about ¼”, set aside. Smear a heaping tablespoon of the roasted garlic guacamole on the tostada. Add some of the chopped chard. Lay two of the pieces of fish on the mounded chard. Spoon some of the solids with a small amount of the oil in the mojo over the fish. Finish with a sprinkling of the coarse sea salt and a squeeze of lime. Serve immediately.