Preheat oven to 375 degrees F. For sauce, in a medium saucepan cook onion in hot oil over medium heat about 3 minutes or until tender. Stir in flour, chili powder, cumin and salt. Cook and stir 1 minute. Stir in tomato sauce and the water. Cook and stir until thickened and bubbly; reduce heat. Simmer, uncovered, 5 minutes, stirring occasionally. Remove from heat.