Potato And Egg Casserole

• 2 potatoes
• 1 egg (hard boiled)
• 1 tomato
• 120 ml sour cream
• 85 gr sharp cheddar (shredded)
• 28 gr butter
• 16 gr flour
• 1 gr parsley (chopped)
• 4 slices green onions
• 1 package graham cracker tarts