Preheat oven to 350°. In a large pot of salted boiling water, blanch cauliflower, 3 minutes. Drain and squeeze cauliflower of water.
Make cheese sauce: In a large skillet, melt butter. Add garlic and cook until fragrant, 1 minute, then add flour and stir until golden, 2 minutes. Add milk and bring to a low simmer, then add cream cheese, whisking until combined. Remove from heat and stir in 1 cup cheddar until melted, then season with salt and pepper.
In a 9”-x-13” dish, add drained cauliflower. Pour over cheese sauce and stir until combined. Stir in all but 1 tablespoon each cooked bacon and green onions until combined, then top with remaining cheddar, bacon, and green onions.
Bake until cauliflower is tender and cheese is melty, 30 minutes.