Melt butter in a saucepan over medium heat; add onion, and sauté 1 minute. Whisk in flour, and cook, stirring constantly, 1 to 2 minutes or until smooth. (Do not brown.) Gradually add chicken broth and cream, whisking until smooth. Stir in salt and ground red pepper. Bring to a boil; reduce heat, and simmer until thickened. Stir in crabmeat and tomatoes; cook just until thoroughly heated.
*1 pound unpeeled large, fresh shrimp, cooked and peeled, may be substituted.