Special equipment: an immersion blender
Combine the whole tomatoes, onion, olive oil, 4 cloves of the garlic, the basil, oregano, salt and pepper in a 2-gallon container or large pot and puree with an immersion blender until smooth.
Heat a large saute pan over high heat until it begins to smoke. Reduce the heat to medium, add the grapeseed oil and the remaining 4 cloves garlic and quickly saute. Add the diced tomatoes and saute for 1 to 2 minutes. Add the pureed tomato sauce and bring to a simmer. Cook for 15 minutes and then remove from the heat. Bring back to a simmer when you are ready to serve.