These are just some thoughts of mine.
The Navi tells us that an angel delivered a pizza to Eliyahu (I Kings 19:6). The term the Navi uses to describe what I refer to as “pizza” is “Ugas Retzaphim”. Ugas Retzaphim might translate as “pavement cake”.
Artscroll translates Ugas Retzaphim as “coal-baked cake.” It seems to me this would be a flour based food baked directly on dying coals, or on a stone heated by coals that have then been moved to one side of the stone. Relief paintings in Egyptian tombs show bread being baked on hot ashes. Bedouins still utilize this technique. People who go camping also bake this way as can be found on some Youtube channels
Ugas Retzaphim can also possibly mean baked on the wall of the oven, as with a Tanur oven.
This is essentially how Neopolitan pizza is made: it is baked on a hot stone next to an intense source of heat.
Similarly, Mishna Baba Kama, Chap. 2, Mishna 3 mentions a Charara, which the Rav Ovadiah from Bartenura translates as a cake baked “on” coals. The term “on” can mean “on top of”, but it can also be translated as “beside” or “above.” The case there in the Mishna refers to a coal stuck to the cake, presumably because the cake was in direct contact with the coal from being on top of it, or right beside it.
Mishna Shabbos, Chap. 1, Mishna 10 mentions Charara Al Gabei Gechalim (Charara on top of coals), and there the Rav Ovadiah from Bartenura translates Charara as pavement cakes. In fact, his explanation in that section makes it evident that the Charara is baking on the earthenware surface of the oven, not directly in contact with the coals despite the Mishna saying on top of.
According to the Jastrow Dictionary גבי can mean “towards”. This seems to fit in well — almost too well — with the way I’d like to understand על גבי גחלים. Thus, the cake was baked on (besides) the coals. This would make eminent sense.
The floor of the oven is first heated with coals. Then the coals are raked to one side. Then the cake is placed on the floor, which bakes from below. Meanwhile the cake is in proximity to the coals which bake the exposed side and top surfaces from above.
As mentioned, this interpretation is the only way the Rav from Bartenura’s commentary seems to make sense: otherwise how can the cake be said to be in contact with the pavement if it’s literally resting on top of the dying coals? Furthermore, how would the top of the cake get done if it’s resting on the dying coals?