How to Make It
Preheat grill to medium-high. On stove, place a steamer basket in a pan over 1 inch of boiling water. Add corn; steam until just tender, 3 to 5 minutes. Season shrimp with 1/4 tsp. each salt and pepper.
Oil grill grates. Grill corn and shrimp, turning, until corn is charred and shrimp are just pink and opaque, 4 to 6 minutes. Add bread; grill, turning often, until charred, 2 to 4 minutes.
Cut kernels from corn in planks. Place bread on plates; smash avocado into bread. Top with arugula, shrimp and corn. Drizzle with 1 Tbsp. oil; season with 1/4 tsp. each salt and pepper. Squeeze lemon over crostini and serve.