Heat oil in a large saucepan over medium-high heat. Add onions and garlic and cook until soft. Add remaining ingredients and cook for 20 minutes. Place mixture in a blender and blend until smooth. Return to the saucepan and cook an additional 15 to 20 minutes, or until thickened. Preheat grill. Reserve 1 cup of the sauce. Brush steaks with olive oil and season with salt and pepper to taste. Place on the grill and grill for 4 to 5 minutes, or until golden brown. Turn the steaks over and continue grilling for 5 to 6 minutes. During the last 2 minutes of grilling, brush the steak with some of the sauce and grill for 1 minute on each side. Remove the steaks from the grill and brush with the reserved barbecue sauce. Serve with the Basil-Marinated Tomatoes on the side.
Basil Marinated Tomatoes:
Combine all ingredients in a medium bowl and season with salt and pepper to taste.