Place the potatoes in a medium pot and fill with cold water. Bring to a boil and cook until just barely fork tender, about 12 to 15 minutes. Don't over cook the potatoes or else they'll get mushy and won't stay on the skewers. They'll continue to cook later on the grill. Drain and set to the side to cool completely.
In the meantime, soak long wooden skewers in water until ready to use. This will prevent them from burning on the grill. You can also just use metal skewers if you have those!
To make the marinade, in a large bowl, whisk together the garlic, oil, soy sauce, red wine vinegar, beef stock and a pinch of salt and pepper; set aside.
To assemble the kabobs, skewer together the potatoes, beef, red onion, tomato, and aji peppers (if using) onto the prepared skewers. Repeat, alternating between each ingredient, until each skewer is filled. Continue until all of the ingredients are used up. Place the kabobs into a large baking dish with high sides. Pour the marinade over the top of the kabobs. Cover with plastic and chill for at least 30 minutes to 1 hour. Rotate the kabobs every 20 minutes to ensure they get evenly marinaded.
In the meantime, to make the aji sauce, put all of the ingredients into a blender and blend until smooth, thinning it out with a splash of water if needed. Give it a taste and adjust seasoning accordingly, adding salt and pepper to taste. Transfer to a container and chill until ready to use.
Heat an outdoor grill or stove top grill pan over high heat. Once very hot, place the kabobs on and grill, turning often, until the kabobs are charred and cooked all around, about 15 to 18 minutes for medium-rare on the steak.
Pour the marinade into a small pot and bring to a simmer over low medium heat. In a small bowl, whisk together the cornstarch with a tablespoon of water. Whisk it into the marinade and cook until bubbly and thickened.
Remove the kabobs from the grill and place on a large platter. Allow to rest for about 5 minutes. Pour the thickened marinade over the kabobs and sprinkle with fresh cilantro for garnish. Serve with white rice and the green aji sauce. Enjoy!