In a medium saucepan, bring the stock to a simmer; keep it at a bare simmer over low heat.
In a medium saucepan, warm the olive oil. Add the onion and stir over moderate heat until softened. Add the rice and cook for 1 minute, stirring to coat it with oil. The Rice will begin to give off a “toasted” smell, but you do not want it to burn.
Add 1 cup of the hot stock to the rice and cook, stirring constantly, until almost all the stock has been absorbed. Add the tomato halves, garlic and another cup of stock and cook, stirring until the stock is absorbed. Continue to add hot stock, 1 cup at a time, stirring until it is absorbed before adding more. The risotto is done when the grains are just tender and bound with a creamy sauce, about 25 minutes in all.
Stir in the butter and half the asiago, chopped parsley, season with salt and pepper. Pour into a large bowl and serve garnish with remaining asiago cheese