Put the pot of water on the stove and allow it to come to a boil. Peel away the thick layers from the onion, the number needed depends on how many hot dog fingers you will create. Also it is important that the onion pieces be wide piece so that we can easily cut them. Set the onion aside. While we are waiting for the water to boil we can prepare the hot dogs. First thing you'll need is a sharp pairing knife that is small and easy to handle. Grab one end of the hot dog and pinch and rip off the tip. This is to create the severed from the hand effect. Feel free to rip off more until you achieve the desired shape and look.
Carefully and working slowly, use the pairing knife to cut out the shape of the nail on the opposite end of the hot dog. You'll want to cut in a couple of millimetres of depth, this is so that the onion nail will fall right into place. Next use the pairing knife to cut, create, and mimic the lines of the knuckle areas. Feel free to use as many or as little as you'd like. I used my own fingers as inspiration for the lines and nail shapes.
By now the water should be boiling. Gently place the hot dogs and onion in the boiling water. Allow the hot dogs to plump-up a bit, but not too much so that they will still look realistic. I left them in the water for about five minutes. Also keep an eye on the onion, it only needs to soften and the colour go almost translucent like a human nail. Once the hot dogs and onion are done place them on a cutting board and allow to cool enough to handle.
If the severed hot dog fingers will be displayed standing up it is a good idea to use wooden skewers. Measure out the length of the hot dogs and cut the skewers leaving enough room for the depth of the container you will display them. Gently grab the boiled hot dog and insert the skewer in the centre slowly working your way up until right before you reach the opposite end. This now should allow you to keep the hot dog standing up. Continue to do the same with the other hot dogs then set aside.
Grab a layer of the boiled onion and begin cutting out the shape for your nails. You can keep comparing the size and shape to the carved out nail part on the hot dog. The best way to cut out the shapes on the onions is with kitchen scissors -- knives take too long and won't work as well. To make the nails stay in place on the hot dogs simply squirt a little bit of ketchup on the carved out nail shape of the hot dog. Gently place the onion nail into the section and arrange as desired. Continue until all of the hot dogs have a nail.
Assemble right before serving. Squeeze some jalapeño ketchup into the serving container or dish where your severed fingers will be displayed. Grab one severed finger hot dog with the skewer and arrange on the serving container, continue until you've created the desired display. Lastly the food dye is added to the areas where we want the bloody effect. I suggest adding droplets of the dye to the nail areas and some of the lines carved out. To make it even more spooky you can gently place slivers of raw onions in some of the slits to make it look like the bone is poking out.
To eat one simply picks up the hot dog and dips it into the ketchup. Depending on how dry the food dye is people may stain their lips or tongues -- make sure to warn them.