Whisk the mayonnaise, sour cream, buttermilk, ranch seasoning, and herbs. Refrigerate until serving.
Heat the oven to 425°F (220°C). Toss the fries with oil, seasoned salt, and garlic powder. Spread in a single layer on a sheet pan and bake 20 to 25 minutes, flipping halfway, until crisp.
Cook the bacon until crisp, drain on paper towels, and crumble.
Toss the cheddar and Monterey Jack together.
Layer half the fries in an oven-safe skillet or on the sheet pan, then half the cheese and bacon. Repeat with the remaining fries, cheese, and bacon.
Bake 3 to 5 minutes until the cheese is melted and bubbling. Broil 60 seconds for browned spots if you like.
Top with jalapeños and green onions. Serve hot with the ranch dip.