Shephards Pie Simplistic

1 lbs ground beef
2 tbsp cooking oil
1/2 cup chopped celery
1/2 cup chopped onion
2 tbsp all purpose flour
1 tsp salt
1/4 tsp pepper
1 cup boiling water
1 beef boullion cube
3 cups mashed potatoes

Combine beef, oil, celery and onion in frying pan. Scramble fry until brown. Stir in flour, salt and pepper. Pour boiling water over bullion cube; dissolve. Pour into meat mixture. Stir until boiling and thickend. Spread into 1 1/2 qt casserole. Cover with potatoes. Bake uncovered in 350F oven for 30 minutes until heated through and potatoes are brown.