Turn on a gas grill to medium to medium-high or light a charcoal fire, letting the coals burn until they are covered with gray ash but still quite hot.
Using a pastry brush or an oil mister, spray one side of the chicken thighs, onion slices and garlic scapes or chives with a light coating of oil. Generously season with salt and pepper.
Lay the onion and chicken oil-side down on the grill and lightly oil and salt the other side. Grill until the onions are browned and soft and the chicken is cooked through (about 4 minutes per side for the onion, about 5 minutes per side for the chicken), turning them half-way through. Place the garlic scapes or their stand-ins on the grill and spread out. Grill quickly until they are just wilted and starting to brown. As the ingredients are done remove them to a baking sheet to let them cool.
Chop the garlic scapes into small pieces, the onion into ½-ince pieces and shred or cube the chicken. Scoop everything into a mixing bowl. Open the can of pickled jalapeños and pour all of the vinegary liquid into the bowl. Separate the vegetables from the chiles, cut the vegetables into small pieces and add to the bowl. Slice as many jalapeňos as you like into ¼ inch strips and combine with the rest of the mixture in the bowl.
To serve, spread about 1 tablespoon of the goat cheese onto the bottom of each tostada, top with chicken in escabeche and pass to your hungry guests.