Rub the chuck roast all over with Italian seasoning, salt, and red pepper flakes.
Heat olive oil in a large skillet and sear the roast on all sides until golden brown, about 3-4 minutes per side.
Place chopped onion and minced garlic in the bottom of the slow cooker, then place the seared roast on top.
Mix together diced tomatoes, beef broth, and tomato paste, then pour over the roast.
Cover and cook on low for 8 hours or high for 4-5 hours until the meat shreds easily with a fork.
Shred or slice the roast, stir it into the cooking liquid, and garnish with fresh basil before serving.