A question about a recipe: Köfte (spiced meatballs)

I have a question about step 4 on the recipe "Köfte (spiced meatballs)" from Janneke Verheij. It says:

"Bake the meat balls and the remaining onion in a small layer of oil until the balls are brown at all sides, they don’t have to be done on the inside yet. Add the tomato cubes, pepper and salt and let the balls simmer with a lid on the pan until done. "