A321 Flight Attendant reported a galley oven began producing heavy smoke during use; triggering a fire alarm in a nearby lavatory. Flight crew returned to departure airport and landed.

Narrative

Approximately 1 hour before landing I was preparing to do the service in FC (First Class). I heard the fire alarm chime four times. I could also hear it in the cockpit. I immediately looked toward the FC LAV; no one was in there. I glanced at the BC (Business Class) LAV and noticed the occupied sign was green. The Captain called me on the interphone stating they have a fire alarm in the BC LAV. I headed toward that LAV to check. The #3 met me halfway into FC cabin and told me she thought the BC oven could be on fire. She explained that the #5 put the cookies in the oven and within a minute there was black smoke pouring out the oven vent in oven 5. She was adamant that there could be a fire in there and asked me (repeatedly) to not open the oven. I called the cockpit to tell them the situation. (That there could be a fire in the oven; but we didn't want to open the door to see.) The Captain explained he was about to [request priority handling] and needed to know if there were flames in the oven. I made sure that he wanted me to open the oven door. He did. I went back to the BC galley and told the #3 and #5 that the Captain needed me to open the oven door. #3 insisted that we do it with her holding a Halon extinguisher. She broke the seal and pulled the pin and on 3 I opened the oven door approximately 3 inches.What I saw-Thick black smoke.Charred oven racks.Burnt food.I did not see flame.I called the cockpit and told them. There were no flames; but everything was charred; and I believed that there had possibly been flames. I based it on what I saw and past experience of seeing burnt food. This looked more sever then past experiences. He said he was [requesting priority handling]. I ran through my briefing questions and was told we would not be evacuating but we had 10 minutes to secure the cabin for landing in ZZZ. I asked him if he was going to make an announcement; or should I? He stated he would make announcement after they went through their checklist. The checklist took a long time and as I was preparing the FC cabin for landing it was hard not explaining to the passengers why we were landing in ZZZ. The Captain made the announcement explaining we were landing in ZZZ and said preparefor landing. I turned the lights to bright (and left them that way). We secured the cabin and took our jump seats. I did my review. We landed and taxied to the gate. Shortly after we were told it was safe to continue on and we left.The following is how we decided as a team to err on the side of caution.The smoke was so thick that it was able to trigger the fire alarm through closed BC LAV door. We could smell smoke throughout the BC and FC cabin and were later told they could smell in the cockpit. My previous experiences with burnt food were dramatically different from what the oven looked like and smelled like when I looked inside. The #5 had used the oven for the meal service and there had been no smoke from that oven which led me and the crew to believe that it was possibly an electrical fire.Suggestions: Put a guard on heating element in oven to protect from food starting a fire.