Preheat oven to 350°.
Cook pasta according to package instructions, being sure to add 1 tbsp. of salt to the boiling water. Drain well & add to large bowl.
In skillet over medium high heat, add olive oil. Add ground beef, onion, salt & pepper. Cook 4-5 minutes until beef is no longer pink and onions are soft.
To large bowl with shells, add tomato paste, tomato sauce and frozen vegetables. Add browned ground beef and onion mixture. Stir well.
Pour mixture into 9x13 baking dish. Top with shredded cheese.
Cover with foil and bake on center rack for 30 minutes.
After 30 minutes, remove cover & bake 10-15 longer until cheese starts to brown.