Preheat oven to 350 degrees F. In a large pot of boiling water, blanch cauliflower, 2 minutes. Drain and squeeze cauliflower of water.
Make cheese sauce: In a large skillet, melt butter. Add garlic and cook until fragrant, 1 minute. Add flour and stir until golden, 2 minutes. Add milk and bring to a low simmer and add cream cheese, whisking until combined. Stir in 1 cup cheddar until melted and season with salt and pepper.
In a casserole dish, add drained cauliflower. Pour over cheese sauce and stir until combined. Stir in all but 1 tablespoon each cooked bacon and scallions until combined, then top with remaining Cheddar, bacon, and scallions.
Bake until cauliflower is tender and cheese is bubbly and thickened, 30 minutes