Preheat oven to 400 degrees Fahrenheit.
Cut potatoes into cubes and add them to a parchment lined baking dish. Drizzle with oil and sprinkle with salt and pepper. Mix to coat and bake for 20 minutes.
Take the pan out of the oven and roughly mash the potatoes with a potato masher. Squeeze the lime over the top and drizzle with the ají amarillo paste, and mix thoroughly. Put the dish back in the oven for an additional 15 to 20 minutes or until the potatoes are cooked through.
While the potatoes are cooking, prep your vegetables and shrimp.
Heat the oil in a large pan over medium hight heat. Cook the ginger and garlic with the shrimp and half of the onions, stirring occasionally for 4 to 6 minutes, depending on how well done you like your shrimp. Add the butter and blend.
Remove potatoes from the oven when cooked, and lay them across the bottom of a platter or shallow bowl. Spread the shrimp mixture across the top of them, and garnish with the rest of the onions, cilantro, and Serrano pepper slices.