Blend marinade ingredients in a small bowl and pour into a baking dish that will hold all 8 onion halves.
Trim the end of onions and cut horizontal (1/4” slices)and place wide side down in marinade.
Marinate overnight covered in refrigerator.
Preheat oven to 375°F. Gently flip the onions over.
Roast uncovered in the oven for 1 hour or until golden brown, basting once or twice during roasting.
Reduce heat to 350°F and top with a little dab of butter on each onion piece and the remaining chopped thyme.
Cook for another 30 minutes or until onions are fork tender and golden brown.