Fill a large bowl with ice water and set it aside. Then bring large pot of water to a boil. Reduce heat to medium so that the water is just simmering; add the salt and shrimp to the pot and poach the shrimp for 2 1/2 minutes. (Don't let the water boil!) Add the garlic to the pot with the shrimp and poach an additional 30 seconds.

Drain shrimp and garlic and then immediately plunge into ice water to stop the cooking.

Once the shrimp is cool, drain it well and then blot with paper towels to remove the excess water. Set aside the garlic.

Chop each shrimp into 2-3 pieces and then put it into a medium bowl. Mince the garlic and combine with shallots, oil, and lemon juice. Toss the dressing with the shrimp and allow it to marinate for at least 4 hours (can be prepared the day before).

When you are ready to make the crostini, remove the shrimp from the fridge and allow it to warm up a bit. Then slice the bread into thin, about 1/4 inch, slices. Lightly brush slices with olive oil. Toast the bread on a grill pan or in the oven until golden brown all over. While the bread is toasting, toss together the avocado, tomato, and scallions in a serving bowl.

Serve slices of bread on a platter alongside the shrimp and the avocado mixture so that people can create their own crostinis.