Put the onions into a small flameproof roasting tin, along with the butter and olive oil. Put the beetroots, parsnips and squash cubes into a separate large roasting tin. Drizzle over the olive oil, tuck in the herbs and season with salt and pepper. Slide both tins into the oven and leave to cook for 25–30 minutes, stirring a few times to make sure they cook evenly.
Back in the kitchen, whisk together the flour, eggs and milk
in a mixing bowl until combined. Season with salt and pepper and set aside.
Spread out the sausages in a high-sided roasting tin 23–25cm square, drizzle over the oil and toss about so they are covered. Bring both the batter and the sausages outside so they are handy by the oven.
Remove the onions from the oven, slide the tin of sausages into their place, and cook for 10 minutes. Keep the veg roasting, rotating the tin and stirring if necessary to keep them cooking evenly.
Finish the gravy while the sausages are cooking. Rest the tin of onions over the hob over a medium heat. Stir in the flour until thoroughly combined with the buttery juices, then slowly pour in the stock, stirring all the time as it thickens. Add the wine and Marmite (if using) and simmer steadily for a few minutes. Keep warm over a very low heat.
Once the sausages have had 10 minutes, remove them from the oven and use tongs to move them into a bowl temporarily – they won’t be particularly crispy yet, but never fear. Check the oven floor temperature; it may have cooled down too much. In that case, remove the vegetables from the oven too and use the metal peel to shift the embers from one side of the oven to the other. Check the floor temperature where the embers were: you want a good feisty temperature – about 250°C is ideal. Slide the vegetables back in, along with the empty sausage tin, favouring the hotter part of the oven for this as you need to get the oil really hot.
After a couple of minutes, slide out the empty tin and quickly pour in the batter while the pan is still really hot – it should sizzle when it hits the oil. Quickly arrange the sausages on top and slide back into the oven as fast as possible. Shut the door, leaving it ajar if there are still live flames in the oven.
Roast for 25–30 minutes – the batter should be puffed up and deeply golden and the vegetables should be tender inside and crisp on the outside.
Serve immediately, with the gravy to pour over both toad and roast veg. A good dollop of mustard wouldn’t go amiss either.