No Fail Lasagna

• 2 egg yolks
• 1 onion (diced)
• 1,400 ml spagetti sauce
• 475 ml milk
• 450 gr beef (lean ground)
• 120 gr oven ready lasagna noodles
• 120 gr ricotta
• 120 gr mozzerella cheese
• 60 gr parmesan cheese
• 55 gr butter