1. Put a steamer basket in a large pot over 1-inch of boiling salted water. Add the asparagus; cover and steam until it is crisp-tender, 6 to 8 minutes.
2. In a small dish, stir together the mint, garlic, lemon zest and pepper flakes.
3. Brush the tilapia with 1 tablespoon oil; season with salt and pepper to taste. Add 2 tilapia fillets to hot pan; cook, turning once, until opaque throughout, 5 to 7 minutes total. Transfer to plates. Repeat with remaining tilapia.
4. Serve the tilapia with the asparagus; garnish with gremolata mixture. Drizzle the asparagus and fish with olive oil, if desired.