Cut the garlic cloves in half lengthways. With a sharp knife, make 6 incisions around the roast and place the garlic halves in the skin.
In the pressure cooker, heat oil over medium-high heat. When it is hot, brown the roast on all sides. Season with salt, pepper and herbs, then add the onion, tomatoes and mushrooms (with juice). Add water and put the lid on the pot, and reduce heat to medium-low heat.
Cook about 30 minutes once it comes to pressure.
Remove the valve, carefully open the cooker and remove the roast to a plate.
Dissolve the cornflour in a little water then pour into the juices in the pan, stirring well. Bring sauce to a boil to thicken. When it reaches your desired consistency, return the roast in the pot to reheat just before serving.