Preheat oven to 350 degrees F and spray a 9x13" baking dish with nonstick cooking spray.
Place a large pot of salted water over high heat and bring to a boil. Once boiling, add the cauliflower florets and blanch for 3-4 minutes. Drain the cauliflower and transfer to a plate lined with paper towels (to absorb some water). Set aside.
Chop the bacon into small pieces and add them to a large saucepan set over medium heat. Once the bacon has started to sizzle, cover with a lid, and cook until golden (or preferred doneness). Don't overcook as bacon will continue cooking in the oven.Remove from heat and transfer bacon bits with a slotted spoon to a plate lined with paper towels. Set aside.
If using butter, wipe the saucepan clean of any leftover bacon grease and add 1 tablespoon to the pan.If using bacon grease, discard all but 1 tablespoon to fry.Return the saucepan to medium heat. Add the chopped onion and saute for 3-4 minutes or until caramelized. Add the minced garlic, stir, and cook 1 minute.
Reduce the heat to medium-low. Add the additional 2 tablespoons of butter to melt. Once melted, sprinkle with flour and whisk until a paste forms. Allow the flour to cook 1 minute.
Slowly pour in the milk, whisking as you pour to fully combined. Pour in the half and half, whisk. Bring to a low simmer for 2-3 minutes before adding the cream cheese. Continue whisking until all of the cream cheese has melted.
Remove from heat. Stir in the greek yogurt, 1 cup of sharp cheddar cheese, and half the bacon. Season the sauce with salt and pepper - to taste.
Transfer the cauliflower to the prepared baking dish. Pour the cheese sauce overtop and gently stir to combine. Top with remaining cheddar cheese and bacon.
Bake for 20-25 minutes or until the cauliflower is fork-tender. Garnish with salt, pepper, and freshly chopped chives.
Add chicken to make this a complete meal!Keto-friendly option: you can either omit the flour entirely, allow the milk to cook down a little more or use a little less milk and a bit more cream cheese and cheddar cheese for a thicker sauce. Another option is guar gum or xanthan gum.Gluten-free option: use the GF version of all-purpose flour or cornstarch as a thickener.Freshly shredded cheese will always melt better and more easily than prepackaged.