This study investigated the effects of four different dietary onion (Allium cepa L.) preparations from the new Allurion variety on body composition, metabolic parameters, hepatic gene expression, and gut microbial activity in healthy male Wistar rats. Fifty 7-week old rats were randomly assigned to five dietary groups and fed semi-purified diets for four weeks: control, untreated onion, osmotically dehydrated onion with a glucose-fructose syrup, and dehydrated onion further subjected to lactic acid fermentation with Levilactobacillus brevis ŁOCK 944 and Lactiplantibacillus plantarum P27. Body weight, feed intake, and body composition were monitored, and blood, liver, and cecal samples were collected for biochemical, enzymatic activity, bile acid, short-chain fatty acid, and gene expression analyses. Dietary onion preparations reduced hepatic expression of lipogenic genes Hif1a, Scd1, and Acly, indicating anti-lipogenic effects. Osmotic dehydration boosted these benefits by suppressing inflammatory and lipogenic genes (Nfkb1, Srebf1, Tlr4) and improving plasma markers of liver function and lipid metabolism. Fermentation had a preparation-specific impact on liver genes that regulate fatty acid oxidation, bile acid metabolism, and inflammation. The percentage of lean tissue decreased in dehydrated onion-fed rats, yet there were no changes in their overall body weight, and fermentation did not further affect their body composition. Cecal enzyme activities and bile acid profiles were modulated, reflecting favorable gut microbial activity. These findings demonstrate that dietary onion, particularly when processed via osmotic dehydration and selective fermentation, can beneficially modulate hepatic gene expression, plasma biochemistry, and gut microbial function, supporting its potential as a functional dietary component to promote metabolic health.