Bacon Cheese Broccoli Gratin

broccoli florets

slices bacon

olive oil

large yellow onion, thinly sliced

sugar

garlic, minced

salt and pepper

butter, plus more for greasing the pan

all-purpose flour

+ ½ cups milk

Dijon mustard

+ ½ cups shredded Havarti cheese, divided

shredded white cheddar cheese

grated parmesan cheese

ground cayenne pepper, optional

chopped green onions