First caramelize your onions. Halve or quarter your onions and cut into thin slices. Melt 1/2 stick butter and olive oil in a medium size, high sided skillet over medium high heat. Add onions, cook 5 minutes until slightly translucent, add salt and pepper (a pinch of cayenne for a kick) then reduce heat to very low. Cook for at least an hour, stirring occasionally, until onions are a gorgeous deep amber color. Remove onions from skillet and set aside.

Fry your bacon to your preference in the same skillet. Remove and place on paper towel to cool. Once it's cooled chop or tear into bite sized pieces.

To this very same skillet (imagine all that good bacon, caramelized onion flavor in there!) over medium/low heat, add 3 Tablespoons of butter and melt. Once butter has melted add the flour and cook together for at least one minute, you want to cook off the flour a bit so your sauce doesn't taste of it. Slowly whisk in your milk, I do about a cup at a time. Be sure to whisk well. After each addition let this mixture bubble and thicken (can take up to 5-10 minutes as you near the end). If need be add a bit more milk, you don't want this sauce to be oh-so thick because you'll be adding a lot of cheese which will naturally thicken it up.

Once you've got a slightly thick, silky sauce, remove skillet from heat and whisk in cheeses, a few cups at a time, until fully incorporated and smooth.

With a wooden spoon, fold in your cooked pasta, then onions, and finally cooked bacon.