Heat oil in a heavy skillet over medium heat. Cook onions slowly until golden brown. Add mustard and vinegar and cook another 2 minutes. Remove from heat and add honey, orange zest and thyme, stir to combine. Let cool.
Preheat grill, Brush the potato slices on both sides with the vegetable oil and season with salt and pepper. Grill the potatoes on each side for 3 to 4 minutes or until just cooked through. Place potatoes in a bowl and fold in onion mixture.