Step #1 Melt butter with oil in large soup pot - add sliced onions & stir to coat - cover pot & cook over moderately low heat for 15 - 20 mins - stirring every once in awhile until onions are tender and translucent.
Step #2 Uncover pot & raise heat to moderately high - stir in the salt & sugar (sugar carmelizes & helps onions to brown) - cook about 30 mins - stirring frequently until onions have turned an even deep golden brown.
Step #3 Lower heat to moderate - stir in the flour and add a bit more butter if flour does not absorb into a paste with the onions.
Step #4 Cook slowly, stirring constantly for 12 mins to brown flour lightly.
Step #5 remove this from heat - pour about 1 c of warmed consomme into onion/flour mixture to mix flour & consomme - add rest of consomme, wine, water, bay leaf & sage - bring to a simmer.
Step #6 Simmer slowly for 30 - 40 mins.
Step #7 If you are not serving right away, let cool, uncovered, then cover and put in the fridge.
Step #8 Reheat when ready to serve - place this in ovenproof soup bowls - top this with a slice of toasted french bread, shredded mozzarella cheese & parmesan cheese.
Step #9 Place under broiler to melt cheeses until bubbly.