Preheat the oven to 200°C (400F)
Wash all the vegetables thoroughly under running water, to remove any dirt. There is no need to peel any of the vegetables.
Cut the onion in half. Place on a non-stick pan with the cut side down and place on high heat until the exposed surface of the onion is burnt. This step is optional, but if done, the stock will have a darker colour and a slightly deeper flavour.
Chop the carrot and the celery into large chunks and cut the garlic bulb in half crosswise. If using the meat, cut it into large chunks.
Place all the veal bones, meat and vegetables in the roasting tray, distributing them evenly. Roast them in the oven for around 1 hour until they are evenly browned on all sides. The time needed for this may vary depending on the size of the bones, so keep checking the oven and adjust the time as needed. Ensure that none of the bones or vegetables are burning.
Gently transfer the roasted bones, meat and vegetables into a large pot. Do this with a pair of tongs or a slotted spoon, to avoid the fat at the bottom of the pan from being transferred to the pot.
Discard all the fat from the roasting tray. Deglaze the roasting tray with a cup of cold water, scraping off all the caramelisation from the bottom of the tray. If needed, you can return the dish to the oven for a few minutes to make the scraping easier.
Add the liquid from the roasting tray to the pot, together with the rest of the cold water. Also add the bay leaf and the thyme. It is important that the water is very cold. It is even better half the water is replaced by ice cubes.
Place the pot over low heat and let simmer for 10 hours. Ensure that the pot is only gently simmering and not boiling. As fat and impurities float to the top of the pot, use a ladle to skim them off. This is usually only necessary for the first couple of hours.
After the simmering is done, use a pair of tongs to lift out as many of the bones as possible from the pot. Then use a ladle to pass the stock through a fine meshed chinois. You can also simply pour the stock through the chinois without using the ladle, but this will result in a more cloudy/less clear stock.
Let the stock drip through the chinois for a few minutes to make sure none is wasted. Do not press the liquid through the chinois, as this will also make the stock cloudy.
A layer of fat will form at the top of the stock. This needs to be removed. It can either be skimmed off using a clean ladle or if the stock is left to cool in the fridge, the fat will solidify and can be simply scraped off using a spoon.