Bring a large pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain.
In a large saucepan, melt the margarine over medium-high heat. Add milk and pesto sauce mix; bring to a boil stirring constantly with a wire whisk until well blended and boiling. Reduce heat and add spinach and mushrooms. Simmer for 3 to 4 minutes, stirring occasionally.
Add cooked pasta, tuna and tomatoes, stirring gently to coat. Cook 3 to 5 minutes until thoroughly heated.