Make the pesto (this can be done ahead): Stick a palmful of almonds, a small handful of parsley, and a heart attack-inducing amount of salt in a food processer until it's nice and crumbly (don't worry about the salt, the pesto will be eaten in small portions!) Beat together about a tablespoon of softened butter with a couple glugs of olive oil until the butter floats around in little clumps. Mix oil mixture and almond/parsley mixture together and add freshly ground pepper. Taste it because it probably needs more salt, then plastic-wrap your container and put it in the fridge for later or continue the recipe.
Preheat the broiler.
If you're using fresh shrimp, shell and take the yucky stuff out, butterfly, then cook, saute, or grill with olive oil and black pepper. If using pre-cooked shrimp, thaw.
Thinly slice the onion, and preserve the circle shape while saute-ing gently in oil just until tender, ~5 minutes.
Slice the bread 1-1.5 inches thick and lay out on an ungreased baking sheet. Brush with olive oil.
Thinly slice tomatoes, cover each crostini with one layer (either one slice from a big tomato or three slices from a grape tomato).
Place a slice of the onion on top of each tomato, lay one (jumbo) or two (small) shrimp on top of the onion. Sprinkle chopped green onion over the tops.
Broil until the bread is nice and toasty, about 10 minutes.
Spoon a small amount of pesto onto each crostini, and serve.
For bonus points, sprinkle with mozerella cheese!