Add the olive oil and butter and sauté the onion, carrots and celery for 8 minutes, or until the vegetables soften. Stir in the fresh thyme and cook for another minute or so. Add the beef and pork to the Dutch oven and cook, seasoning with salt and freshly ground black pepper and breaking the chunks of meat up with a wooden spoon, until there is no pink left in the meat. At this point, you can drain some of the fat out of the pot if desired. Tilt the pot up, pushing the meat to one side so the fat collects on the other side. Remove all but 2 tablespoons of the fat with a spoon or turkey baster.