Preheat the oven to 400 degrees F.
Slice the bread into 1/2-inch thick slices. Lightly brush the bread slices with the 3 tablespoons of olive oil. Arrange the bread on a baking sheet fitted with a rack and bake until lightly toasted, about 8 minutes. Remove from the oven and let cool before topping with the crab mixture.
In a large bowl whisk together the lemon juice, 1/4 cup of olive oil and salt and pepper, to taste. Set aside.
Add the yellow pepper, the avocado, shallot and the crabmeat to the lemon vinaigrette and toss gently, so as to not break up the crabmeat. Top the bread slices with about a tablespoon of the crab mixture and arrange on a serving platter.