Heat the oven to 425°F and line a sheet pan with parchment paper.
Soak the potatoes in cold water 15 minutes, drain, and pat completely dry.
Toss with oil, cornstarch and salt, spread in a single layer, and bake 35 to 40 minutes, flipping once, until deep golden and crisp.
While the fries bake, cook the bacon in a skillet over medium heat, 8 to 10 minutes, turning occasionally, until it shatters when bent, then drain on paper towels and crumble.
Pile the hot fries onto a platter and scatter the cheddar jack over them while hot.
Top with the crumbled bacon and diced tomato.
Drizzle with ranch dressing and finish with green onion just before serving.
Good to Know
Cook the bacon the morning of and leave it at room temperature; it crisps right back up under the warm fries without reheating.