Boil the halved Brussels sprouts in plenty of salted water until tender, about 5-7 minutes..
Cook the bacon in the butter over a very gentle heat until it is starting to brown (10-15 minutes).
Add onions, and cook until translucent. Add garlic and thyme, and sweat for a few seconds more, until fragrant. Stir in flour.
Still over a very gentle heat, gradually pour in milk, whilst stirring briskly. Bring slowly to the boil, still stirring, Simmer for a few minutes, until thickened, stirring now and then. Add thyme, and season to taste.
Make topping: Combine crumbs, cheese, olive oil and pepper in a small bowl.
Assemble: Grease 6 individual dishes. Distribute Brussels sprouts between them. Pour over bacon Béchamel, and top with crumble mixture. Bake in the preheated oven until bubbling and golden on top.