Preheat the oven to 400°F (200°C). Position the oven rack in the lowest position. Line a rimmed baking sheet with aluminum foil. Use a small paring knife to carefully make 1-inch deep incisions all over the cauliflower and 1 1/2-inch deep X in the base stem of the cauliflower.

In a medium bowl, mix together the mayonnaise, cheddar cheese, and Parmesan cheese.

Place the cauliflower on the prepared baking sheet, stem side down, and slather the cheesy mayonnaise all over the top of the cauliflower. Place the bacon slices next to the cauliflower. Place on the lowest oven rack, reduce the heat to 375°F (190°C) and roast for 20 minutes. Carefully flip the bacon. Roast until the bacon is crispy, another 5 to 10 minutes. Remove the bacon and drain on a paper towel. Continue roasting the cauliflower until the paste has become a crust, the surface is dry and lightly browned, and a paring knife easily pierces the cauliflower with no resistance, 30 to 35 minutes more. If a little of the cheese mayo slides off during cooking, gently scoop it up, put it back on.

Remove the cauliflower and let it cool for 10 minutes. Crumble the bacon, then serve the cauliflower with the green onion and bacon.