Heat the oven to 200 C / 400 F with racks in the upper and lower thirds. Brush both sides of the 8 tortillas with 1 tbsp oil, sprinkle with 1/4 tsp salt, and arrange 4 on each of two 33 x 46 cm / 13 x 18-inch sheet pans.

Stand each corn ear upright in a wide bowl and slice off the kernels with a chef's knife, which gives about 2 cups / 300 g. Dice the onion, mince the jalapeño and garlic, quarter the tomatoes, chop the cilantro, and juice the limes for 3 tbsp total.

Bake the tostadas for 10 to 12 minutes, swapping the pans and flipping each tortilla at the 6-minute mark, until the edges are deep golden and the shells feel stiff when tapped. They keep crisping as they cool.

While the tostadas bake, heat a 26 cm / 10-inch cast-iron skillet over high heat for 2 minutes. Add 1 tsp oil and the corn in one layer, and leave it undisturbed for 2 minutes. Stir, then repeat for 6 to 8 minutes total, until about a third of the kernels have blackened spots. Constant stirring steams the corn and leaves it pale. Tip it into a large bowl.

Mash the beans in a bowl with the garlic, cumin, 1 tbsp lime juice, 3 tbsp water and 1/4 tsp salt, leaving some beans partly whole. Scrape the mash into the same skillet over medium-low heat and stir for 3 minutes, until it's thick and spreadable like soft hummus.

Let the corn cool for 2 minutes, then add the avocado, tomatoes, onion, jalapeño, cilantro, 2 tbsp lime juice and 1/2 tsp salt. Fold gently with a spatula so the avocado stays in chunks, then taste and add more salt or lime if needed.

Spread 2 to 3 tbsp warm beans on each tostada, then top with shredded romaine, about 1/2 cup salsa and crumbled cotija. Serve within 10 minutes of topping, with lime wedges on the side.