Unfortunately I cannot find data specific to Tanzania but I have gone ahead and looked up the data for Eastern Africa as a whole using FAOSTAT (Food and Agriculture Organization of the united Nations Statistics Division) http://faostat3.fao.org/home/E
The average yield of onions in Eastern Africa in 2013 was 58980.3 hectograms per hectare, which converts to 5.9 metric tons per hectare. Based on this data. one acre of land in Eastern Africa in 2013 produced an average of 2.4 metric tons of onions. Please see the attached images for other years.
The yield you obtain is obviously based on many factors, including soil quality and climate so it is very difficult to give you a concrete answer. Please feel free to ask any other questions you may have about growing onions
FAOSTAT output for dry onion yield in Eastern Africa 1995 to 2013 (Hg/Ha)
Table of onion yields for Eastern Africa between 1995 and 2013, converted to metric tons per acre
Line graph of onion yields in Eastern Africa between 1995 and 2013 (Hg/Ha), FAOSTAT