In small saucepan, saute onions until tender. (Use dollop of margarine and water.) In 3 litre saucepan, heat margarine over medium heat until bubbly and hot. Add flour, mustard and stir quickly to combine thoroughly. Stirring constantly, gradually add milk, continue to stir and cook until smooth. Add broth mix and pepper and mix well. Reduce heat to low and cook, stirring occasionally, until mixture is thick, 5 to 10 minutes. Add cheese and cook, stirring until melted. Add chicken and vegetables and continue cooking until chicken and venerable are heated through. Preheat oven to 400 degrees. spray casserole dish with non-stick cooking spray and pour chicken mixture into dish. Separate each biscuit into two layers of dough. Arrange on top of chicken mixture. Bake until biscuits are golden, 7 to 10 minutes.