Coarsely grind coriander seeds in a spice grinder or a mortar and pestle. Place onion in a bowl of ice water for 5 minutes. Drain, then let dry on paper towels in refrigerator.
Meanwhile, heat 1 teaspoon oil in a small saucepan until hot. Add pumpkin seeds, and cook, swirling pan constantly, until plumped and toasted, 2 to 3 minutes. Drain on paper towels, and season with 1/4 teaspoon salt. (Pumpkin seeds can be prepared 1 day ahead.)
Heat a large nonstick saute pan or skillet over medium heat. Brush both sides of bread slices with 1 teaspoon oil total. Toast bread in skillet until brown, 1 to 2 minutes per side.
Whisk together lime zest and juice with remaining teaspoon oil. Add onion, carrot, basil, half the coriander, and 1/4 teaspoon salt, and toss well. Top each slice of bread with half the avocado, and season with remaining 1/4 teaspoon salt and remaining coriander. Top with salad and pumpkin seeds.
When shaving with a mandoline, the trick is to use the protective guard at the right time. As the piece of produce gets smaller and your fingers draw nearer the blade, firmly press the teeth of the guard into the vegetable, making sure it's secure. Keep a good grip on the mandoline at all times.