In a large skillet, add olive oil and onions and sauté over medium-high heat for 2 to 3 minutes or until translucent. Add turkey, season well with salt & pepper and for about 7-10 minutes. Add ketchup, mustard, cayenne, brown sugar and tomato paste. Reduce heat and simmer for 10 to 12 minutes. Finish with a splash of vinegar and season it once more.
Serve on hamburger bun.