Ham and sweet potato gratin

Ingredients

• 500 ml finely chopped leaks
• 2 chopped onions
• 115 g butter
• 60 ml flour
• 5 ml onion powder
• 5 ml powdered garlic
• 750 ml milk 2% M.F
• 1 kg sweet potatoes, cut thin
• 1 kg cooked ham, diced
• 250 ml grated cheddar cheese
• Softened butter
• Pepper