In a cast iron skillet (or an oven safe skillet), over medium heat, add olive oil, ground beef, onion, celery, salt and pepper. Cook for 10-12 minutes, stirring occasionally until beef is browned and veggies are soft.
Add diced tomatoes, balsamic vinegar, garlic, basil, and oregano. Cover and continue to cook over medium heat for about 8-10 minutes, stirring occasionally, until tomatoes are soft. Add two cups of spinach and cook for an additional 2-3 minutes, covered, until spinach is cooked down.
Add tomato sauce, parmesan cheese, crushed red peppers and stir to combine. Cover and cook for 15 more minutes to let the flavors develop into the sauce.
While the pasta sauce is cooking, cook ziti pasta according to package instructions for "al dente".
Preheat oven to 350 degrees
Toss cooked ziti into your finished sauce and stir to combine. Top with grated mozzarella cheese.
Bake in the preheated oven for 15-17 minutes, until top is nice and golden. Then remove from the oven and let cool for a few minutes. Enjoy!