In a medium skillet, melt the butter over medium heat. Add the corn and onions. Cook, stirring occasionally, until the onions are tender, 5 to 6 minutes.
In a medium bowl, combine the cornmeal mix, flour and salt. In a small bowl, whisk together the buttermilk and eggs, and add to the flour mixture, stirring just until the dry ingredients are moistened. Stir in the corn mixture and cheese.
In a medium Dutch oven, pour oil to a depth of 3 inches; heat to 360 degrees F. In batches, drop the batter by heaping tablespoons into the hot oil. Cook until golden brown, 2 to 3 minutes per side. Drain on paper towels. Serve immediately.