Mama Moe's Scalloped Potatoes

Ingredients

• 4 large potatoes, peeled and thinly sliced
• 1 onion, thinly sliced
• 3 tablespoons butter
• 3 tablespoons all-purpose flour
• 2 cups milk
• 1 cup shredded cheddar cheese
• Salt and pepper to taste
• 1/4 cup grated Parmesan cheese