Heat a large skillet over medium-high heat. Add 1 tablespoon oil to pan; swirl to coat. Add onion and bell pepper; sauté 5 minutes. Add potatoes; cover, reduce heat to medium, and cook 8 minutes or until potatoes begin to brown, stirring occasionally. Stir in salt and black pepper.
Combine milk and flour, stirring with a whisk. Add milk mixture to pan; cook 3 minutes or until thick and bubbly, stirring frequently. Remove pan from heat. Stir in yogurt and cheese. Spoon mixture into an 11 x 7-inch baking dish coated with cooking spray.
Place cornflakes in a medium bowl; drizzle with butter and remaining 1 1/2 tablespoons oil, and toss to coat. Sprinkle cornflakes over potato mixture. Bake at 350° for 35 minutes or until bubbly around the edges and topping is crisp.