Preheat oven to 180°C Cook pasta in a large saucepan of boiling, salted water, following packet directions, until tender. Drain.
Heat oil in a large frying pan over medium heat. Add onions, garlic and celery. Cook, stirring, for 2 minutes or until onion is soft. Add tomatoes. Cook for 2 minutes. Add spinach, pesto and milk. Cook, stirring, for 2 minutes or until spinach just wilts. Add pasta to pan with tuna (undrained). Season with salt and pepper. Toss gently to combine.
Spoon mixture into an 8 cup-capacity ceramic baking dish. Top with cheese. Bake for 20 minutes or until cheese is melted and golden. Stand for 5 minutes before serving.