Rinse and pat the Firm White Fish dry, then cut into roughly 1/2-inch cubes so the citrus can penetrate evenly.
Place the fish in a nonreactive bowl and pour enough Lime Juice over it to just cover; season with a pinch of Kosher Salt and a few grinds of Black Pepper.
Cover and refrigerate the bowl for 15–30 minutes (shorter for very small cubes, up to 45 minutes for larger pieces) until the fish turns opaque and feels firm to the touch.
While the fish cures, dice the Roma Tomato, thinly slice the Red Onion, mince the Serrano Chile (remove seeds for less heat), dice the Cucumber, and chop the Cilantro.
Drain most of the curing liquid from the fish (save a tablespoon or two if you like extra brightness), then gently fold the fish together with the tomato, onion, chile, cucumber, and cilantro.
Cut the Avocado into cubes and fold it in last to keep it from mashing; adjust seasoning with additional Kosher Salt and Black Pepper as needed.
Squeeze a little fresh juice from a Lime over the ceviche to brighten flavors, then spoon onto Tostadas and garnish with extra cilantro leaves or lime wedges.
Serve immediately for the best texture and flavor; refrigerate any leftovers and consume within 24 hours.
