In a large stockpot, sauté the onion and carrot in the grapeseed oil until lightly browned. Add the veal or beef trimmings and continue to sauté until the meat is well browned. Add the garlic and red wine. Reduce the wine until there is only 100 ml (3½ fl oz) of liquid left, then add the stock, tomatoes and thyme. Reduce the stock on a medium simmer until the stock has halved in quantity. Strain through a fine conical strainer lined with muslin cloth. You should now have a demi-glaze (half reduced stock).
To make a glaze, return the demi-glaze to a clean pot and reduce over high heat until you have the required viscosity for glazing and saucing meats. Generally from 4 litres (140 fl oz) of demi-glaze, you will need to reduce it to somewhere between 800 ml (28 fl oz) and 500 ml (17 fl oz/2 cups) to achieve a glaze. The further you reduce, the heavier the glaze.