Whisk together eggs, milk, Marmite, and thyme until all ingredients are fully incorporated. In a separate bowl whisk together flour, baking powder, and a pinch of salt and pepper.
Whisk dry ingredients into wet ingredients then toss sliced onions into batter using a spatula or your hands to gently crush the onions. Allow to sit for 30 minutes.
Preheat the oven to 300[°] and place a foil lined baking sheet in the oven. In a large pot over medium high heat, heat 3 inches of oil until a piece of onion dropped in the pot sizzles (it should register about 325°F on an instant read or deep frying thermometer). Using a spatula drop approximately 1/4 cup sized fritters into the oil. Cook, turning often until the fritters are golden brown, about 4 minutes.
Transfer fritters to a rack set in a rimmed baking sheet and place cooked fritters in the oven to keep warm and repeat until all the batter is used.
Serve hot garnished with chopped parsley and a drizzle of Marmite if desired.