Granted, it helps that there are less than ten ingredients, which I will actually list for you this time. I know, I'm actually going to give you a recipe? What trickery is this? Don't worry, I'm still not giving you quantities or specifics. All is right in the kitchen.
First- in a well-insulated pan, like a dutch oven, sauteuse, or soup pot, melt the butter over low heat. I'm talking like a 4 on the dial. Cut your onions in half then slice them like you were making rainbows (thanks, Lis, for that description) and add to pan. DO NOT DICE THEM. I used about a stick and a half of butter for 6-7 medium onions. Sprinkle some salt over the onions. Keep the stove on low and stir until the onions are coated in butter. Let sit, stirring occasionally. If your onions start to burn, lower the heat. You do want some colouring, however, so don't freak out if they start browning on the bottom of the pan. Depending on the quantity of onions this process could take up to a couple of hours. Mine took about an hour to get to the colouring you see in that terrible picture above. That is what you are going for. When they look dark enough add some pepper and the stock. Turn the heat up so the soup boils, then transfer into oven-proof bowls. Or, in my case, cast iron coquettes.