Preheat the oven to 180 degrees. Cook the macaroni in plenty of salted water until al dente. Dice the onion and pork belly and fry in a pan until translucent.
Grease a baking dish with butter. Drain the pasta and layer alternately with the bacon and onion mixture in the pan, then top with bacon.
Whisk the flour and eggs together, season with salt and pepper and pour evenly over the casserole. Sprinkle breadcrumbs over the casserole and put flakes of butter on top. Put in the oven on the middle rack and bake for about 45 minutes, until the casserole is nice and golden brown.