Melt butter in a large saucepan. Add in onions and garlic. Saute over medium heat for 3 minutes. Sprinkle on flour and stir. Cook an additional 3 minutes. Whisk in milk and bring to a boil. Reduce heat to medium low and add in tomato sauce, salt, basil and oregano. Cook on medium low for 15 minutes, stirring occasionally.
Meanwhile, cook tortellini and broccoli in boiling water for 3 minutes. Drain.
Add tortellini and broccoli to tomato sauce mixture. Stir in cheddar cheese.
Put mixture into a 9x13 inch casserole dish and sprinkle Parmesan cheese on top. Bake for 20 minutes at 350°F.