Season the tri-tip with kosher salt and black pepper. Grill over high heat until the thickest part reaches 125–130°F for medium-rare. Rest 15 minutes, then chop.
Slice the russet potato into thin fries and rinse under cold water until the water runs clear. Dry thoroughly with paper towels. Fry at 350°F until golden brown and crispy, then immediately toss with kosher salt and chopped cilantro.
Mash the avocados with lime juice and kosher salt until smooth but still slightly chunky.
Preheat a comal or griddle over medium-high heat. Add cooking oil, a layer of shredded quesillo, then top with the tortilla. Sear until melted and slightly charred, then remove from heat. Top with guacamole, chopped tri-tip, salsa, and crispy fries. Optionally, add pico de gallo and/or sour cream.
