Heat oil in a medium nonstick skillet over medium-low heat. Add onion, garlic, and salt. Cook 25 to 30 minutes, stirring often, until browned. Add mirin; bring to a boil. Reduce heat, and simmer, uncovered, 5 minutes or until liquid is absorbed.
Place onion mixture, beans, and remaining 3 ingredients in a blender or food processor; process 1 minute or until smooth, scraping sides of bowl occasionally. Cover and chill at least 2 hours (mixture will thicken as it chills).