In a frying pan, melt butter and sauté onion until translucent (about 3-4 minutes). Add red wine to deglaze the pan, then add Dijon mustard, brown sugar, pepper and 1/2 tsp (2 mL) of the salt. Simmer until liquid has reduced to just a few tablespoons and onions have caramelized. Set aside to cool.
Brush flatbread with olive oil and sprinkle the remaining 1/4 tsp (1 mL) sea salt on top. Heat in frying pan over medium heat, 1-2 minutes per side, until soft. Repeat with second flatbread.
Place both flatbreads on a baking sheet or pizza stone to cool. Spread each with half the onion reduction, then place half the cheese on top. Bake at 375°F (190°C) for 6-8 minutes, until cheese has melted. Cut each flatbread into 6 pieces. Makes 2-4 servings.