How can I achieve an even heat across a large paella pan? I just used a 20" paella pan to make a large paella for 10 people.  The pan covers 4 burners.  The problem that I had was that the center of the pan never reached the temperature of the parts of the pan under the burners. If I raise the gas levels enough for the center to get to a boil, the outside areas get too hot and burn.  Difficult to make an even paella.  Any suggestions?  I have looked for, but not found any heat diffusers that are large enough for this pan.
The oven can work, however, if you have the option, I would suggest outdoors over fire.  Paella is a festive occasion, and this adds to the spectacle.  For your 20 inch pan, you could construct a temporary brick fire pit sort of like the one below, as a friend of mine regularly does.  Alternately, you can place your pan over a charcoal grill, provided it fits. Mine, for example, fits into the opening of my Big Green Egg smoker/grill.  If you don't have those options, using the stove top will mean a constant rotation of the pan.  That gets messy and potentially dangerous once liquid is added. So, I would agree with the oven recommendation...if it fits.