Beef Zucchini Casserole

• 1 onion (sliced)
• 700 gr beef (lean ground)
• 500 gr thinly zucchini about (sliced)
• 160 ml ketchup
• 40 gr parmesan cheese (grated)
• 28 gr butter
• 8 gr flour
• 3 gr salt
• 1 gr pepper