It's important to note they aren't going to brown in just 10-15 minutes. They take more towards 45 minutes - 1 hour. Sometimes even longer! But if they aren't browning, here are some possible reasons.  #1: The onions are cut too thick.  #2: You have too many onions in the pan #3: You need the right amount of heat. Make sure that you keep the temperature between medium and medium low. #4: You aren't cooking them in the right pan. Use a cast iron or stainless steel pan to produce the best caramelized onions. Nonstick pans and enameled cast iron work well too, it just will slow down the caramelization process and take longer for the onions to caramelize.