Make these tostadas on the grill, in the summer, when produce is most flavorful and everyone will enjoy them to the fullest.
Heat grill to high; lightly oil grates. In separate piles, arrange zucchini, mushrooms, and scallions on a baking sheet. Drizzle with 2 tablespoons oil; season with salt and pepper. Working in batches if necessary, remove each pile of vegetables from baking sheet. Grill, turning once, until lightly browned and tender, 2 minutes for scallions, 6 minutes for mushrooms, and 8 minutes for zucchini. Return all vegetables to baking sheet. Slice mushrooms into 1/2-inch-wide strips. Set aside.
Brush tortillas with remaining tablespoon oil; grill, turning frequently, until browned and very crispy, about 2 minutes. Place 1 tortilla on each serving plate; cover evenly with mushrooms and zucchini. Using kitchen shears, snip scallions over vegetables. Top with salsa, and sprinkle with feta; serve with lime wedges, if desired.