So good that friends will beg you for the recipe!
Stir in salt, red onions and sweet onions. Cook 35 minutes, stirring occasionally, until onions are sticky and almost syrupy. (Onions will be translucent.)
Arrange 4 large oven safe bowls or crocks on a rimmed baking sheet. Fill each bowl 2/3 full with hot soup.
Top each bowl with 5-6 toasted croutons, 1/2 cup Gruyere cheese and 1/2 of the Asiago cheese
Broil 4-5 minutes, or until bubbly and golden brown. Keep a close eye on them!