On medium heat, add 1½ T. olive oil to skillet and add plums facedown (skin facing up); Let plums cook down until softened, but still slightly firm (approx. 10-12 minutes). Remove from heat.
In separate skillet, saute sliced onion in 2 T. olive oil over medium heat until onions soften (about 4-5 minutes). Season onion with course seat salt and freshly-ground pepper (approximately, 1 ½ tsp each, or to taste), stirring onions frequently
Add plums to the onion skillet, gently cooking together over medium heat; Add 2 T. each of dijon mustard and brown sugar, gently blending into the onion-plum mixture. Cook down another 5-6 minutes, or until plums begin to become a thick sauce
Set plum-onion mixture aside, allowing plums to cool.
Gently pull the plum skin to remove; discard
Pre-heat oven to 325°F
Lightly tenderize chicken breasts and top with sea salt and pepper (1/2 tsp. each per chicken breast)
Brush top side of breast with Dijon mustard
Bake for 20-25 minutes (I use a convection oven and breasts are near-fully cooked at 20 minutes)
Remove chicken from oven and top breasts with plum-onion mix; Bake another 10 minutes
*Serve with organic fingerling potatoes (tossed with olive oil, sea salt, ground pepper, and Herbs de Provence).