Dissolve yeast and sugar in lukewarm water in a large bowl.
Add 3 cups flour, softened butter, onion soup mix and salt to yeast mixture.
Beat with an electric mixer set at medium-high for 2 minutes. Add remaining flour and beat until blended.Let rise, covered, until almost double in size, about 30 minutes.
Generously butter two 71/2 x 4 inch loaf pans.
Stir down the dough. Spoon into prepared pans.Let rise, covered until dough reaches tops of pans, about 1 1/2 hours.