In a heavy bottomed saucepan, over medium-high heat, add the grapeseed oil. When the oil is hot add the onion and saute until translucent, 3 to 4 minutes, stirring throughout. Deglaze the pan with wine. Once the pan has been deglazed add the stock, tomatoes, rosemary, and parsley.
Reduce the sauce until it is reduced by 40 percent, about 15 to 20 minutes. Once the sauce has reduced, remove from heat and pour through a chinois or cheesecloth into a bowl. Return the sauce to the pan and stir in butter to finish the sauce. Sauce can be held warm until serving or stored in a covered container in the refrigerator.