While the water boils for the pasta, heat up a sauté pan with oil to sweat the chopped onions, garlic, and tomatoes. Add the corned beef and break the meat apart as it cooks until everything is combined. Set that aside.
Drain the water off of the pasta. Dump the cooked penne into the sauté pan with corned beef. Stir everything around.
In a small pot, make the cheese sauce. Melt the butter, sprinkle the flour and whisk to form a roux. Add the milk, stir for a bit then add the shredded cheese. Season with a sprinkle of salt and pepper. Keep whisking until the cheese has melted and the sauce is a little thick and bubbly. Pour the cheese sauce in the pasta and corned beef pan. Stir to incorporate everything together. Preheat the oven to 355°
Transfer the whole thing into a casserole dish.
Sprinkle the remaining shredded cheese and panko bread crumbs on top. Cover with foil then stick it in the oven to bake for 15-20 minutes.