In a large saucepan, cook onion and garlic in butter over medium heat until tender. Combine sour cream and flour until smooth; gradually add to onion mixture. Stir in the broth and pepper. Bring to a boil. Reduce heat; simmer, uncovered, for 2-3 minutes or until thickened. Remove from the heat; stir in 1/2 cup of cheese.
Place crab in a bowl; stir in 1/2 cup sauce (I usually do just a little bit more, just until moistened). Spoon equal amounts on tortillas; roll up tightly. Place seam side down in an 11 inch x 7 inch baking dish coated with cooking spray. Top with remaining sauce.
Cover and bake at 350 degrees for 30 minutes. Uncover; sprinkle with remaining cheese. Bake 5 minutes longer or until cheese is melted. Let stand for 5 minutes. Top with tomato and green onion.