In a large frying pan, heat the oil and melt the butter over medium-high heat. Sear the chicken breasts, turning once. Place in an ovenproof dish and brush with mustard.
In the frying pan, brown the onion until tender.
Deglaze with white wine and reduce by half.
Add the sugar and thyme. Cook until the onion is caramelized and the liquid has almost completely evaporated. Season to taste.
Cover the chicken with the caramelized onions, spinach and cheese.
Continue baking in the oven for 12 minutes.
Serve immediately with pasta.
Tip from the chef
This recipe would be as delicious with turkey breast.