While the spaghetti squash is cooking, sauté the ground beef in a large skillet until it is browned.
Once the meat is browned, add the onion, pepper, and garlic to the skillet with the meat and cook until softened, about 5 minutes.
Add the diced tomatoes (with their juices), tomato sauce, and Italian seasoning blend to the skillet and stir to combine. Bring to a simmer and simmer for 15 minutes.
Once the spaghetti squash is cooked, use a fork to remove the strands from its shell. Add the spaghetti squash to the skillet and use tongs to incorporate the squash with the tomato mixture. Taste and add salt and pepper as needed.