Step 1: Preheat your oven to 425°F (220°C). This will ensure that your potato slices get crispy and delicious.
Step 2: Wash the russet potatoes thoroughly and slice them thinly using a sharp knife. Aim for about 1/8 inch thickness for optimal crispiness.
Step 3: In a large mixing bowl, toss the potato slices with olive oil, kosher salt, and ground black pepper until well-coated.
Step 4: Spread the seasoned potato slices in a single layer on a large baking sheet. Bake in the preheated oven for 20-25 minutes, flipping halfway through, until they are golden brown and crispy.
Step 5: While the potatoes bake, combine the cooked and shredded roast beef with taco seasoning in a mixing bowl. Mix well to ensure the beef is evenly coated.
Step 6: Once the potatoes are crispy, remove them from the oven. Layer half of the potato slices on a serving platter or a large baking dish. Top with half of the seasoned shredded beef and half of the cheddar and mozzarella cheeses. Repeat with the remaining potato slices, beef, and cheeses.
Step 7: Return the assembled nachos to the oven and bake for an additional 10-15 minutes, or until the cheese is melted and bubbly.
Step 8: Once out of the oven, top your nachos with Pico de Gallo, dollops of sour cream, fresh cilantro, and slices of avocado. Serve with lime wedges and salsa on the side for extra flavor.