In a large skillet cook onions in oil over med-low heat until they are tender. Add broth and pepper. Bring to a boil. Remove from heat. In a dutch oven melt butter. Stir in flour until smooth. Add milk slowly. Bring to a boil cook and stir for 1-2 minutes or until thick. Reduce heat. Add cheeses and onion mixture. Cook and stir until heated through and cheese is melted.