Preheat oven to 375. Heat oil in a large saucepan over medium-high. Add beef, sausage, onion, carrots, celery and kosher salt. Cook, breaking up meat into bite-size pieces, until vegetables soften and meat has browned in places, about 10 minutes. Add garlic, oregano, and red pepper flakes; cook 1 minute. Add tomato paste and cook 2 minutes. Puree tomatoes in a blender or food processor; add to pan. Bring to a boil, then reduce heat to low and simmer, stirring occasionally, until reduced slightly, about 15 minutes. Remove from heat.