Begin by peeling and cutting the russet potatoes into ½-inch thick fries. Soak them in a large bowl of cold water for at least 30 minutes.

Heat your vegetable oil in a deep pot to 325°F (160°C). Carefully add the soaked fries in batches, frying them for about 5-7 minutes until soft.

Once the fries are cool, increase the oil temperature to 375°F (190°C). Fry again in batches for 2-3 minutes until golden brown.

After frying, toss the hot fries with salt and set them aside.

Thinly slice the yellow onions and sauté in a skillet with butter and olive oil, sprinkling with sugar, salt, and black pepper for 30-45 minutes until deeply caramelized.

In a mixing bowl, whisk together mayonnaise, sweet pickle relish, ketchup, yellow mustard, white vinegar, garlic powder, onion powder, and cayenne pepper if desired. Refrigerate for at least 30 minutes.

On a serving plate, pile the crispy fries and top with melted cheese. Use a broiler or microwave to melt the cheese if desired.

Add the caramelized onions on top of the cheesy fries and drizzle with the prepared secret sauce. Serve immediately.