Drain tomatoes and onions very well. Thaw a package of frozen corn. Melt butter and brown garlic in butter. Stir in flour. Add milk, salt, pepper, basil, oregano and sugar. Cook until thick. Stir in 1/2 cup cheese. Add the well-drained tomatoes to the cheese sauce. Cook until thickened again. Put the corn and onions into a 1 1/2 - or 2-quart casserole. Pour cheese-tomato sauce over vegetables. Top with the other 1/2 cup grated Cheddar cheese. Bake at 350° for 50 minutes, uncovered.