Cook bacon 5 to 7 minutes over medium heat until browned & crispy. Drain & discard fat & place bacon on paper towel to drain then chop. (Or cook as whole slices in 400F oven on a wire rack in a baking sheet in a single layer, for about 20 minutes or until browned & crisp.)
Trim cauliflower of leaves & remove core. Slice cauliflower & separate into medium- large sections (some will fall apart but that’s okay), removing any big stems.
Preheat oven to 375F. Grease a medium-large baking dish.
Heat oil in a large frying pan over medium-high heat. Add chicken in 2 batches & cook until browned all over; remove from pan.
Cook leek & garlic in the same pan for 2 to 3 minutes or until leek is softened. Add squash & chicken & stir to combine. Cover, reduce heat to medium & cook for 5 minutes, stirring once during cooking.
Add cream, thyme, salt, pepper & nutmeg; stir gently to combine. Spoon half the mixture into the baking dish. Sprinkle with half the cheese & half the bacon. Spoon remaining mixture over cheese. Arrange cauliflower over cheese. Sprinkle with remaining bacon & cheese. Place on a baking sheet (in case there’s any cheesy overflow). Bake, uncovered, for 45 to 50 minutes until the top is golden brown & cauliflower is tender. Sprinkle with extra thyme if desired.