Heat @olive oil{1%Tbsp} in #large skillet{}. Add thinly sliced @onion{1%medium} and cook for ~{20%minutes}, stirring frequently.

Stir in @brown sugar{1%Tbsp}, @salt{1/4%tsp} and @pepper{1/4%tsp}. Add @Bourbon{1%Tbsp}, scraping up brown bits in bottom of skillet. Stir 1.5 tsp @butter{1.5%tsp} into mix.

Spread remaining butter on @sourdough bread{8%slices}.

Top each slice with @cheddar cheese{6%oz} and @Gruyère cheese{6%oz} and onion mixture.

Grill 3 to ~{4%minutes}.