Preheat oven to 425°F. Line two baking sheets with aluminum foil. Set aside.
In a small bowl combine the salt, black pepper, garlic and onion powder, and cayenne pepper. This is the "house seasoning" for the nachos. Set aside. Using a mandolin or a knife, slice the potatoes about ½ of an inch thick, in circles. Place on the prepared baking sheets on a single layer. Drizzle with the olive oil and sprinkle with the house seasoning blend. Bake for 25 to 30 minutes, turning halfway through baking, until golden brown and crispy.
Meanwhile, heat a skillet over medium heat. Remove the sausage from the casing and place in the heated skillet. Using a wooden spoon breakdown the sausage and allow to brown and crisp up, about 5 minutes. Throw in the diced onion and minced garlic. Stir and cook for another 3 minutes until the onions are translucent. Add the red pepper, if using. Lower the heat to low and cook for 2 more minutes. Transfer to a bowl or plate.
In the same skillet, set over medium heat, melt the butter and sprinkle in the potato starch or all-purpose flour. Whisk together and cook for a few seconds. Slowly stream in the warmed milk while continuously whisking. Season with the house seasoning and allow to thicken. Cook for a few minutes, whisking constantly, until the sauce has thickened. Turn off the heat and add the grated Irish cheddar cheese. Whisk until completely melted and smooth. Rewarm the potatoes in the oven for a few minutes.
Place the potatoes on a platter, bowls, or a skillet. Top with cheese sauce and browned sausage and onion mixture. Sprinkle with diced tomatoes, sliced scallions, sliced pickled jalapeños, avocado (mashed with lime juice and seasoned with house seasoning), sour cream and cilantro. Eat right away. Enjoy!