Spread the frozen fries in a single layer on a lined baking sheet so they don't steam each other. Bake according to the package directions, flipping them halfway through for even browning. I always wait for deep golden edges before pulling them out, since undercooked fries turn mushy once the toppings go on.
While the fries bake, cook your bacon in a skillet over medium heat until it's fully crisp. Drain it on paper towels to remove extra grease, then crumble it into small pieces once it's cool enough to handle. Crispy bacon matters here because soggy bacon gets lost under melted cheese.
Grate the cheddar and Monterey Jack fresh while the fries and bacon finish cooking. Freshly grated cheese melts smoother and clings to the fries better than the bagged pre-shredded kind. Keep the two cheeses separate or mix them in a bowl, whichever is easier for you.
Once the fries are done, pile them onto an oven-safe platter or skillet instead of spreading them thin. Sprinkle the shredded cheese generously over the top, making sure it reaches down between the fries. Add the crumbled bacon right on top of the cheese so it gets warmed through.
Slide the loaded platter under the broiler for about 2 minutes, watching closely the whole time. You want the cheese fully melted and just starting to bubble, not browned or burnt. Broilers work fast, so I never walk away during this step.
Pull the fries out and immediately scatter the green onions and pickled jalapeños over the melted cheese. Drizzle the ranch dressing generously across the top, and add a dollop of sour cream if you're using it. Serve right away while everything is still hot and gooey.