For this tostada, flank steak, zucchini, and onions are marinated in a cilantro, garlic, and jalapeno puree, then grilled.
Halve an onion crosswise, and place one half in the bowl of a food processor. Add jalapeno pepper, garlic, 1 cup cilantro, olive oil, and 1/2 teaspoon salt; puree about 1 minute. Cut remaining onions into 1/4-inch-thick rounds.
Preheat oven to 400 degrees. In a bowl, coat flank steak, onions, and zucchini with cilantro puree. Cover; marinate at least 30 minutes.
Place tortillas on baking sheets, two at a time, and place in oven until dry and golden brown, 10 to 12 minutes. If baking two trays at a time, rotate halfway through baking. Repeat with remaining tortillas.
Preheat a grill, or set a grill pan coated with olive-oil cooking spray over medium-high heat until hot. Place steak on grill; cook about 10 minutes. Turn and cook until medium rare, about 10 minutes more. Remove from heat; set aside. Place zucchini and onions on grill. Cook until tender and browned, 8 to 12 minutes; turn as needed.
Thinly slice beef. Toss lettuce with 1/2 cup cilantro. Layer each tortilla with tomatoes, lettuce, zucchini, beef, onions, and radishes. Garnish with remaining 1/2 cup cilantro; squeeze half a lime over each tostada, and sprinkle each with a pinch of salt.