Ham And Cheese Casserole

• 450 gr thinly potatoes (sliced cooked)
• 325 gr center ham (diced cut)
• 120 ml skim milk
• 120 ml skim milk yogurt
• 45 gr monterey jack cheese (shredded)
• 30 gr green onions with tops (sliced)
• 30 gr swiss cheese (shredded)
• 20 gr flour
• 14 gr margarine
• 3 gr cornstarch