Prep the oven and pan. Heat your oven to 425°F (220°C).
Lightly grease a 9x13-inch baking dish or similar casserole pan.
Cook the bacon. In a skillet over medium heat, cook chopped bacon until crisp, 6–8 minutes. Transfer to a paper towel–lined plate. Reserve 1 tablespoon bacon fat for extra flavor, if desired.
Roast the cauliflower. Toss florets with olive oil, salt, and pepper on a sheet pan.
Roast for 18–22 minutes, stirring once, until edges are lightly browned and the centers are just tender.
Make the creamy base. In a bowl, whisk together sour cream, cream cheese, heavy cream, ranch seasoning, and the reserved tablespoon of bacon fat (optional) until smooth. Taste and adjust salt and pepper.
Combine with cheese. Stir in 1 cup of shredded cheddar and the Monterey Jack. Fold in half of the cooked bacon and half of the green onions.
Assemble the bake. Add roasted cauliflower to the casserole dish.
Pour the creamy ranch mixture over it and toss to coat evenly. Spread into an even layer.
Top and bake. Sprinkle the remaining 1/2 cup cheddar over the top. Bake at 375°F (190°C) for 15–18 minutes, until bubbling around the edges.
Broil for color. Switch to broil for 1–2 minutes to lightly brown the cheese.
Watch closely to prevent burning.
Finish and serve. Top with remaining bacon, the rest of the green onions, and parsley. Let rest 5 minutes, then serve warm.