Sift the flour into a large mixing bowl and add the beaten egg and milk, and mix to form a batter.
Mix in the thyme, rosemary, salt and pepper and set aside for at least 4 hours in the fridge; overnight is best.
Cut the onions for the gravy in half lengthways, then peel and slice into 3-5mm (¼-⅛") thick half-moon shapes.
Add the butter for the gravy to a small (15-16cm or 6") saucepan (not non-stick) and melt it over a medium-low heat.Add the onions and cook over a low-medium heat for 45 minutes, stirring every 5 minutes or so. If the onions start to darken too quickly, reduce the heat.
Heat the vegetable stock until it is boiling and add the dried mushrooms, then turn off the heat and allow them to soak for 10 minutes.
Preheat your oven to 220°C or 430°F. Whilst the oven is heating up, place a 20cm (8") shallow baking tin in there with the cooking oil.
Peel the swede and cut it into 1.5-2cm (½¾") cubes.
Peel the parsnip and cut it into 1.5-2cm (½¾") cubes.
Add the diced parsnip, swede and carrots to the baking tin, toss to coat in oil and then return it to the oven for 10 minutes.
Peel the halved red onion and then cut it into quarters, taking care not to trim off too much of the root. You want the onion to hold together.
When the vegetables have been roasting for 10 minutes, remove the pan from the oven and pile the vegetables up away from the edges of the pan, then add the red onion wedges.
Pour over the Yorkshire pudding batter and place the pan in the oven, and cook for 30-35 minutes. DO NOT OPEN THE OVEN UNTIL THE TOAD IN THE HOLE IS COOKED!
10 minutes before the toad is cooked, blend the stock and dried mushrooms, and it will form a nice gravy-like texture.
Add the tomato puree to the onions and stir for 1-2 minutes.
Pour the mushroom gravy over the onions and turn the heat up to medium.
Stir in the marmite and treacle, feel free to adjust the amounts of these to your liking and then add salt if required; it should not be.
Simmer the gravy until the toad in the hole is ready to serve.
Notes
If you are not vegetarian try cooking this in a hard animal fat like lard, beef dripping or duck fat, it is incredible!