On separate baking sheets, arrange bread and vegetables; brush both sides with oil and sprinkle with salt and pepper.
Prepare barbecue (medium-high heat) or preheat broiler. Grill or broil bread until crisp, about 2 minutes per side if grilled, and 1 to 2 minutes per side if broiled. Grill or broil vegetables until golden and just tender, about 4 minutes per side if grilled, and 3 to 4 minutes per side if broiled. DO AHEAD: Can be made 2 hours ahead. Let stand at room temperature.
Arrange bread, vegetables, and trout on large platter. Garnish with fennel fronds. Serve, allowing guests to assemble crostini.
Available at some supermarkets and at specialty foods stores and delicatessens.