Step 1Build a medium-hot fire in a charcoal grill or heat a gas grill to medium-high (alternatively, heat a grill pan over medium-high heat). Put the tomatillo on the grill, and cook, turning to evenly char, until blackened with blistered pockets, about 5 minutes. Transfer to a cutting board and let stand until cool enough to handle.
Step 2Very finely chop the charred tomatillo and transfer to a large bowl. Using a Microplane zester, finely grate the garlic, ginger and the lime’s zest directly on top. Halve the lime and squeeze in the juice, then add the olive oil, cumin, chile and cilantro. Stir to combine and season with salt.
Step 3Make Ahead: The chermoula can be refrigerated for up to 1 day.
Step 1Combine the oil and chiles in a blender and purée until smooth. Pour through a fine-mesh sieve into a liquid measuring cup and discard any solids. You should have two-thirds cup drained oil.
Step 2Whisk the egg yolk in a small bowl. While whisking, add the serrano oil in a slow, steady stream until the mixture emulsifies. If it breaks, whisk in a teaspoon of warm water. Whisk in the lime juice, then season with salt.
Step 3Make Ahead: The aioli can be refrigerated for up to 5 days.
Step 1Halve the onion lengthwise then cut it crosswise into very thin half-moons. Put the onions in a glass pint jar or nonreactive container.
Step 2Bring the vinegar and chile to a boil in a small saucepan, then pour over the onions. Add the hibiscus flowers and turn to coat. Seal the jar or cover and refrigerate overnight.
Step 3Make Ahead: The onions can be refrigerated for up to 1 month.
Step 1Build a medium-hot fire in a charcoal grill or heat a gas grill to medium-high (alternatively, heat a grill pan over medium-high heat). Drizzle the asparagus with just enough olive oil to lightly coat (about 1 tablespoon) and sprinkle with salt. Grill, turning to evenly char, until bright green and almost tender, 6 to 8 minutes. Transfer to a cutting board and cut into 1-inch lengths at an angle. Transfer to a bowl with the chermoula and gently fold to evenly coat.
Step 2Divide the aioli among the tostada shells and spread to the edges. Top with the asparagus and its chermoula, then sprinkle over the crumbled cheese, pickled onion, and herbs. Serve immediately.