In a medium bowl sir together the mayonnaise, buttermilk, thyme, garlic, Worcestershire sauce, half a teaspoon of salt and half a teaspoon f pepper and set aside.
Peel the onion, remove the root and slice into half an inch thick wedges.
In another large bowl mix together the flours, one and a half teaspoons of salt and one teaspoon of pepper and add the beer and stir well to combine.
Heat the oil in a wide bottomed pan to 350F.
Working in batches, evenly coat the green beans and onions in the batter letting the excess drip off.
Fry for 2-3 minutes until golden and crip then set aside on kitchen paper to absorb any excess oil.
Sprinkle with sea salt and serve immediately with the buttermilk sauce.